Plant Hybrids #1

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Polyploidy & Hybridization
In San Diego County
   
 San Diego County Oaks 

Table of Contents
  1.    How To Make Polyploid Plants
  2.    Parthenocarpic Seedless Fruits
  3.    Mule: The Classic Sterile Hybrid
  4.    Other Interesting Animal Hybrids
  5.    Chimeras With Different Tissues
  6.    Some Unusual Plant Hybrids
  7.    Tangelo: Tangerine X Grapefruit
  8.    Polyploid Varieties Of Apples
  9.    Delicious Stone Fruits (Prunus)
10.    Triploid Seedless Watermelons
11.    Cultivated Mustard Hybrids
12.    Polyploid Grains For Breads
13.    The Hybrid Easter Lily    
14.    Natural Clarkia Hybrids
15.    Natural Brodiaea Hybrids
16.    Natural Oak Hybrids
17.    Natural Pine Hybrids
18.    Natural Sage Hybrids
19.    Natural Penstemon Hybrids     
20.    Chitalpa tashkentensis
21.    Arabis Triploid Pollen
22.    References

1. How To Make A Fertile Polyploid Hybrid

To produce a tetraploid plant, the alkaloid colchicine is applied to the terminal bud of a branch. All the cells in the developing branch will be tetraploid (4n) with four sets of chromosomes. This includes cells of the stem, leaves, flowers and fruit. Gametes (egg and sperm) produced by a flower on this tetraploid branch will be diploid (2n) with two sets of chromosomes. A flower on the normal diploid (2n) branch will produce haploid (n) gametes containing one set of chromosomes.

Stages of mitosis in the formation of tetraploid cells. The original mother cell is diploid (2n). During anaphase the chromatids separate and move to opposite ends of the cell. Colchicine causes the dissolution (depolymerization) of protein microtubules which make up the mitotic spindle in dividing cells. This leaves the cell with twice as many single chromosomes (four sets rather than two). When this cell divides, each of the two daughter cells will have fours sets of chromosomes, a total of eight chomosomes per cell. [Note: Spindle poisons such as colchicine are used to prevent tumor cells from dividing in certain chemotherapy treatments.]

Autumn Crocus: The Source Of Colchicine
A Generalized Explanation Of Cell Division


2. Origin Of Parthenocarpic (Seedless) Fruits

The botanical term parthenocarpy refers to the development of the ovary of a flower into a fruit without fertilization. [The biological term parthenogenesis refers to the development of an egg without fertilization.] Fruits that develop parthenocarpically are typically seedless. Some seedless fruits come from sterile triploid plants, with three sets of chromosomes rather than two. The triploid seeds are obtained by crossing a fertile tetraploid (4n) plant with a diploid (2n) plant. When you buy seedless watermelon seeds, you get two kinds of seeds, one for the fertile diploid plant and one for the sterile triploid. The triploid seeds are larger, and both types of seeds are planted in the same vicinity. Male flowers of the diploid plant provide the pollen which pollinates (but does not fertilize) the sterile triploid plant. The act of pollination induces fruit development without fertilization, thus the triploid watermelon fruits develop parthenocarpically and are seedless. Most bananas purchased at your local supermarket came from sterile triploid hybrids. The fruits developed parthenocarpically and are seedless.

Close-up view of fleshy, berrylike (baccate) banana fruits. The small black dots inside are the remnants of aborted ovules that did not mature into seeds. Since this fruit develops on a sterile plant without fertilization it is termed parthenocarpic. The following cross shows one plausible origin of the seedless banana:

The cultivated banana is often listed in botanical references as Musa x paradisiaca (Musaceae), although it is actually a complex hybrid derived from two diploid Asian species, M. acuminata and M. balbisiana. Common cultivated bananas are usually triploid (3n) with three sets of chromosomes. [Note: The word "set" is defined here as one haploid set of chromosomes.] If A represents one haploid set of chromosomes from diploid M. acuminata (AA) and B represents one haploid set of chromosomes from diploid M. balbisiana (BB), then hybrid bananas have three sets of chromosomes represented by AAB, ABB or another 3-letter (triploid) combination of A's and B's. Like seedless watermelons and red grapes, bananas are sterile and do not produce mature seeds. [Sometimes you can find aborted ovules inside the fruit that appear like tiny black dots.]

In the formation of gametes during normal meiosis, homologous chromosomes must pair up with each other during synapsis of prophase I. Like other odd polyploids (with 3 sets of chromosomes), bananas are sterile and seedless because one set of chromosomes (A or B) has no homologous set to pair up with during synapsis of meiosis. Therefore meiosis does not proceed normally, and viable gametes (sex cells) are not produced. Since banana fruits (technically berrylike ripened ovaries) develop without fertilization they are termed parthenocarpic. Without viable seeds, banana plants must be propagated vegetatively (asexually) by planting corms, pieces of corms or sucker sprouts.

See Synapsis During Prophase I of Meoisis

Parthenocarpy can be induced by growth hormones such as gibberellic acid (GA3) in which the ovaries mature without fertilization. Grape cultivars such as 'Thompson Seedless' are treated with gibberellic acid to order to produce larger fruits with longer internodes. The bunches have wider spaces between the grapes and better air circulation, reducing their susceptibility to fungal diseases and rotting within the bunch. Contrary to some references, 'Thompson Seedless' grapes are not parthenocarpic because fertilization does occur, but the ovules fail to develop into seeds within the maturing fruit.

In cultivated figs, parthenocarpy generally refers to the development of the ovaries of female flowers within the syconium into drupelets without fertilization. The syconium is the structure that you typically associate with an edible fig fruit; however, it is really a flask-shaped structure lined on the inside with numerous unisexual flowers. The actual botanical fruits (called drupelets) develop within the syconium. Since the entire syconium enlarges and ripens into a juicy, sweet morsel, it is often referred to as a fruit. The female flowers are pollinated by a tiny female fig wasp that enters the syconium through a pore called the ostiole. According to W.B. Storey (Advances in Fruit Breeding, 1975), there are 2 genetically determined forms of parthenocarpy: stimulative and vegetative. Stimulative parthenocarpy involves the insertion of the wasp's ovipositor down the stylar canal into the ovary of short style flowers. It can also be induced by blowing air into the syconium, or by spraying the syconium with a plant growth regulator. The mature drupelets may contain a wasp (if an egg was laid in the ovary) or it may be empty. Vegetative parthenocarpy involves the formation of drupelets without any external stimulation, and is responsible for the hollow drupelets inside common figs such as "black mission," "kadota," and "brown turkey." [Some authors use the term parthenocarpy to describe the ripening of seedless fig syconia on the tree without any pollination or fertilization.]

See Genetics Of Triploid Bananas
Formation Of Seedless Watermelons
See Sex Determination In Common Figs
See Wayne's Word Article About Grapes


3. Hybrid Between a Horse and a Donkey

Mule: A sterile hybrid between a horse and a donkey.

The mule is a hybrid between a female horse or mare (2n=64) and a male donkey or jackass (2n=62). Since the mare contributes 32 chromosomes in her egg and the jackass contributes 31 chromosomes in his sperm, the mule has a diploid number of 63. Male and female mules are typically sterile because the horse and donkey chromosomes differ in number and they are not homologous. Therefore, the horse and donkey chromosome doublets fail to properly pair up with each other during synapsis of meiosis I. In fact, one horse doublet lacks a donkey doublet to pair up with. By the way, if the mother is a donkey or jennyass and the father is a stallion, the resulting sterile hybrid is called a hinny.

The mule is an unusual animal because it has an odd number of chromosomes (2n=63) that is not divisible by two. The haploid number of a mule is not 31.5 because you can't have half of a chromosome in the gametes. But there is another way to get an animal or plant with an odd number of chromosomes called aneuploidy. If the chromosome number in the sperm or egg is more or less than the normal number of chromosomes in a haploid set, the resulting offspring (called an aneuploid) may have a chromosome number that is not exactly diploid (2n). Trisomy (2n+1) occurs when an individual has an extra copy of a chromosome. Examples of trisomy in people are Down's syndrome (three #21 chromosomes) and several sex chromosome aneuploidies caused by extra X or Y chromosomes, including Klinefelter's Syndrome (XXY), Trisomy X Syndrome (XXX) and the XYY Male Syndrome (XYY). Monosomy (2n-1) is caused by an individual missing one chromosome. Turner's Syndrome is a human female who received only one X chromosome from one parent and no X or Y chromosome from the other parent. Usually the cause of aneuploidy is nondisjunction during meiosis I or meiosis II, in which the sperm or egg carries extra or fewer chromosomes. In animals, autosomal monosomies and trisomies (abnormal numbers of autosomes) are usually detrimental and often fatal. In duckweeds (Family Lemnaceae), Mr. Wolffia's favorite plant family, the number of chromosomes in one haploid (1n) set is 10; however, polyploidy is common in the family, including 3n=30, 4n=40, 5n=50, 6n=60, 7n=70, and 8n=80. There are also good examples of aneuploidy in the duckweeds Landoltia punctata, Lemna minuta and L. minor, including adult (sporophyte) individuals with 36, 42, 43 and 44 chromosomes. Aneuploid duckweeds often lack vigor and are sterile. Odd polyploids such as 3n, 5n and 7n are also sterile. Can you guess why?

See The Duckweed Family Home Page


4. Some Other Interesting Animal Hybrids

The liger is the result of a cross between a male lion (Panthera leo) and female tiger (Panthera tigris). The hybrid between a male tiger and female lion is a tigon. Some ligers have grown to be much larger than lions, and are the undisputed largest cats on earth. [Images from slides donated to the Life Sciences Department at Palomar College.]

The cockapoo is a popular hybrid between the cocker spaniel and poodle.

According to John Roach (National Geographic News 16 May 2006), DNA evidence confirms a natural hybrid between a female polar bear and a male grizzly. Apparently, grizzly bears have migrated north into Canada's western Arctic, and occasionally enter the range of polar bears. The male hybrid's white fur was interspersed with brown patches. It also had long claws, a concave facial profile, and a humped back--all grizzly characteristics. Unfortunately, this unique hybrid bear was killed by a hunter who thought it was a polar bear when he pulled the trigger. The common name for this hybrid is "polargrizz." I have not substantiated this remarkable discovery in a peer-reviewd scientific journal.


5. Chimeras

In ancient Greek mythology, the chimera was a fabulous monster having a lion's head, a goat's body and a serpent's tail. Some so-called "hybrid" animals mentioned in news articles are actually chimeras. They are produced by combining genes and cells of two species. With the modern age of biotechnology the list of possible chimeras is unlimited. Some genetically engineered chimeras are so bizarre that the combination of species used in the chimera would never be genetically compatible for in vitro or in vivo fertilizations. Hybridoma chimeras are made fusing the cells of two species. One such hybridoma cell line is referred to as a "geep" because it contains the genes of a goat and a sheep. Specific monoclonal antibodies used in cancer research are produced by plasma cells that have been fused with myeloma (cancerous) cells. Because of their tumor component, these antibody-producing cells are "immortal" and continue to divide and produce the same type of antibody. Hybridoma cells have now been created that produce monoclonal antibodies that attack and bind to the poison oak allergen. This research opens the door for an effective new treatment for a potentially serious dermatitis. A combination of the common cold and the polio virus has shown great promise in curing brain cancer. Similar combinations with HIV show promise with other diseases. There is also the potential of producing killer viruses for biological warfare by combining known pathogenic organisms. For example, could airborne strains of HIV be made by combining this deadly virus with influenza, or a deadly form of smallpox combined with Ebola? There are rumors of other super-chimeras such as veePox, a combination of smallpox and Venezuelan encephalitis.

'Pinachée,' a parthenocarpic variety of Ficus carica with alternating yellow and green striped syconia and stems. Variegated plants are often referred to as chimeras. Chimeras are organisms composed of two genetically different types of tissue. Chimeras may result from the fusion of cells or tissue. In variegated plants, a mutation in the chloroplast DNA often results in the loss of chloroplasts. Consequently, this mutant tissue has no green pigment and no photosynthesis. Colorless tissue in variegated plants may also be caused by viruses.

See Nondisjunction Diagram For Biology 100


6. Some Unusual Plant Hybrids

See The Astonishing Filbert-Rubber Tree Hybrid
Amazing Hybrid Between Common & Creeping Fig


7. Tangelo: A Hybrid Between Tangerine & Grapefruit

The citrus family (Rutaceae) contains some of the world's most delicious fruits, including numerous hybrid crosses between species. The popular tangelo grown in San Diego County is a hybrid produced by crossing a grapefruit (C. x paradisi) with a tangerine (C. reticulata). The grapefruit is sometimes called a pomelo, and this explains the blending (portmanteau word) of tangerine and pomelo. Actually, the grapefruit is a hybrid produced by crossing the shaddock or pummelo (Citrus maxima) with a sweet orange (C. sinensis). The shaddock is a large, thick-skinned, tropical citrus fruit up to six inches (15 cm) in diameter that is occasionally sold in supermarkets.

A. Grapefruit (Citrus x paradisi); B. Mandarin Orange or Tangerine (C. reticulata); C: Tangelo (Citrus x tangelo). The Tangelo is a hybrid produced by crossing a Grapefruit (C. x paradisi) with a Tangerine (C. reticulata). The grapefruit (C. x paradisi) is another hybrid between the Shaddock or Pummelo (C. maxima) and the Sweet Orange (C. sinensis). The lime (Citrus aurantiifolia) is a spiny tree native to tropical Asia. There are several varieties including the small, round "bartenders lime" ('Mexican') and the larger California cultivar 'Bearss.' The 'Mexican' cultivar is also grown in the Florida keys and is the source of delicious "key lime pie." The kumquat (Fortunella margarita) is a citrus relative native to tropical Asia. Bigeneric hybrids between limes and kumquats are called "limequats" and are placed in the hybrid genus (x Citrofortunella).

According to Citrus Pages by Jorma Koskinen (2009) the genus Fortunella is taxonomically invalid.
Therefore, all of the varieties and hybrids of kumquats should be listed under Citrus japonicaThunb.

See Fruits Of The Citrus Family (Rutaceae)


8. Diploid and Polyploid Varieties of Apples

According to Apples: A Catalog of International Varieties by Tom Burford, there are 17,000 varieties of apples! Most of the apples grown commercially are probably diploid (2n), although there are many triploid varieties. For example, 'Gravenstein' apples are triploid with a chromosome number of 51 (3n=51). They are produced by the union of a diploid egg (2n=34) and a haploid sperm (n=17). This is accomplished by crossing a tetraploid plant (4n=68) with an ordinary diploid plant (2n=34). Because the triploid (3n) varieties are sterile, they must be propagated by grafting, where the scions of choice cultivars are grafted to hardy, pest-resistant root stalks.

Apples are mentioned throughout most of recorded human history. The generic name Malus is derived from the Latin word malus or bad, referring to Eve picking an apple in the Garden of Eden; however, some biblical scholars think the fig, and not the apple, was the forbidden fruit picked by Eve. One of the earliest records of any fruit eaten by people of the Middle East is the common fig (Ficus carica). Remnants of figs have been found in archeological excavations dating back to the Neolithic era, about 1000 years before Moses. The fig is also the first tree mentioned in the Bible in the story of Adam and Eve. There are some scholars who think the apricot is a more likely candidate because it was an abundant fruit (along with figs) in the ancient Palestine area. Other interesting tales about apples include Johnny Appleseed, William Tell, Sir Isaac Newton, and Apple Computers.

Assorted cultivars of apples (Malus domestica): A. 'Fuji,' B. 'Granny Smith,' C. 'Braeburn,' D. 'Red Delicious,' and E. 'McIntosh.'

Ancient Fig Trees Of The Holy Land
Fruits Of The Rose Family (Rosaceae)


9. Delicious Stone Fruits Of The Genus Prunus

The rose family (Rosaceae) includes many economically-important fruit trees known as stone fruits in the genus Prunus. Each species has many vamed cultivated varieties (cultivars). Botanists have moved some of these species into separate genera, including Amygdalus (peach) and Armeniaca (apricot). Some examples of stone fruits are fuzzy-skinned peaches (P. persica syn. Amygdalus persica), smooth-skinned peaches called nectarines (another variety of P. persica), plums (P. domestica), apricots (P. armeniaca syn. Armeniaca vulgaris), and cherries (P. avium and P. cerasus). Like apples and pears, there are hundreds of cultivated varieties. These fruits are technically referred to as drupes because they consist of an outer skin or exocarp, a thick, fleshy middle layer or mesocarp, and a hard, woody layer (endocarp) surrounding the seed. The part of these fruits that is eaten by people is the mesocarp layer and also the exocarp if you don't bother to peel them. The woody endocarp layer protects the seed and probably aids in the dispersal of drupaceous fruits by hungry herbivores. In wild plants with drupes, the seeds can pass through the entire digestive system of grazing animals and be planted in new locations. The almond (Prunus amygdalus syn. Amygdalus communis) is also a drupe with a green exocarp and thin mesocarp surrounding the pit. When you crack open an almond to get the seed, you are actually cracking open the endocarp layer.

Some species of Prunus have been artificially crossed to produce some unusual hybrids. The peachcot (Prunus persica x P. armeniaca) is a hybrid between the peach and apricot; the cherrycot (P. besseyi x P. armeniaca) is a hybrid between the cherry and apricot; the plumcot (P. domestica x P. armeniaca) is a hybrid between the plum and apricot. Some of these hybrids have many different named cultivars, depending on which varieties of stone fruits have been crossed together. In addition, hybrids often retain more characteristics of one parent and are given special names. For example, some cultivars of plumcots are called "pluots" because these resemble plums more than apricots. Plumcots called "apriums" resemble apricots more than plums.

Plumcots, a delicious hybrid between the plum (Prunus domestica) and apricot (P. armeniaca). Since this cultivar resembles its plum parent more than its apricot parent, it is called a "pluot." Plumcot cultivars that resemble apricots more than plums are called "apriums."

See Stone Fruits Of The Rose Family (Rosaceae)


10. Genetics Of The Triploid Seedless Watermelon

Modern varieties of the watermelon are derived from the native African vine Citrullus lanatus (syn. C. vulgaris). Cultivated for thousands of years in the Nile Valley, this species still grows wild in the arid interior where it supplies native people with water during drought seasons.

According to R.W. Robinson and D.S. Decker-Walters (Cucurbits 1997), wild populations of C. lanatus var. citroides, which are common in central Africa, probably gave rise to domesticated watermelons (var. lanatus). Wild, ancestral watermelons (var. citroides) have a spherical, striped fruit, and white, slightly bitter or bland flesh. The pale flesh tastes like the rind of a typical watermelon. They are commonly known as the citron or citron melon, not to be confused with the "citron" Citrus medica of the Citrus Family (Rutaceae). The citron is also called "preserving melon" because the fruit rind is used in preserves, jellies and to make pickles or conserves. Because of its high pectin content, it is added to fruit juices to make them jell more rapidly. One plant may produce up to 100 fruits, which are commonly fed to livestock. Citron melons become weedy vines in cultivated melon fields of North America, and are unmistakable among other cucurbits because of their pinnatifid (pinnately dissected) leaves. The citron is naturalized in the Cape Region of Baja California, along with the curious teasel gourd.

See Citron Growing Wild Along RR Tracks In San Diego County

Modern triploid watermelons (with three haploid sets of chromosomes) are unable to produce viable gametes during meiosis, and much to the delight of growers, their ripened melons are seedless. [Note: The word "set" is defined here as one haploid set of chromosomes.] They are produced by crossing a tetraploid (4n) seed parent bearing 2n eggs with a diploid (2n) pollen parent bearing haploid (n) sperm. Tetraploid plants are produced by treating the terminal buds of diploid plants with colchicine, causing the chromosome number of the meristematic cells inside to double. The haploid (n) sperm from a pollen grain from the male flower of the 2n parent fertilizes the diploid (2n) egg inside the ovule of a female flower on the 4n parent. The resulting 3n zygote develops into a 3n embryo inside a seed. Planting this seed will yield a 3n watermelon plant bearing 3n seedless watermelons. The following illustration shows this cross resulting in a triploid watermelon plant:

The triploid seed will germinate and grow into a triploid plant bearing triploid male and female flowers, but the flowers will not produce viable sperm-bearing pollen or eggs because of the odd number of chromosome sets (3). With three sets of chromosomes, one set will not have a matching (homologous) set to pair up with during synapsis of prophase 1 of meiosis. This synaptic failure results in gametes that are not viable, therefore double fertilization inside the ovule does not occur and an embryo-bearing seed is not typically formed. When you buy seedless watermelon seeds, you get two kinds of seeds, one for the fertile diploid plant and one for the sterile triploid. The triploid seeds are larger, and both types of seeds are planted in the same vicinity. Male flowers of the diploid plant provide the pollen which pollinates (but does not fertilize) the sterile triploid plant. The act of pollination induces fruit development without fertilization, thus the triploid watermelons are seedless.

Not all watermelons are red. This triploid, seedless variety has sweet, yellow flesh.


11. Hybrid Vegetables In The Mustard Family (Brassicaceae)

Brussels Sprouts, a cultivated variety of Brassica oleracea. Brussels sprouts are grown for their tender, leafy buds along the main stem that resemble miniature heads of cabbage. This unusual variety was apparently selected from a mutant cabbage plant originally discovered in a European garden in the mid 1700s. Kohlrabi, another variety of B. oleracea, has an enlarged basal stem above the ground. The following vegetables are also varieties of B. oleracea: Cabbage (leafy head), kale (non-heading leafy sprout), collards (nonheading leafy sprout), broccoli (immature inflorescence and stalk or peduncle), and cauliflower (immature inflorescence). All of these varieties have 9 chromosomes per haploid set (n=9), with a diploid number of 18 (2n=18). A hybrid between broccoli and cauliflower is called broccoflower (see next photo).

Varieties of Brassica oleracea: A. Broccoli, B. Broccoflower and C. Cauliflower. Broccoflower (B) is a hybrid between broccoli (A) and cauliflower (C). In all three vegetables you are eating the immature inflorescence (flower buds and pedicels), and in the case of broccoli, the main inflorescence stalk or peduncle.

A: Turnip (Brassica rapa), a vegetable with an edible taproot and leaves (turnip greens); C: Cabbage (Brassica oleracea), a vegetable with edible leafy heads; B: Rutabaga (Brassica napobrassica), a vegetable with edible taproot and leafy greens. The rutabaga is a fertile tetraploid hybrid between the turnip (n=10) and cabbage (n=9). Since the original diploid rutabaga (2n=19) has 10 turnip chromosomes and 9 cabbage chromosomes that are unequal in number (10 + 9) and not truly homologous, the diploid hybrid is sterile. It cannot undergo normal pairing of chromosome doublets during synapsis of meiosis I, and therefore cannot produce viable gametes or seeds. The fertile tetraploid hybrid (4n=38) has 2 haploid sets of turnip chromosomes (10 + 10) and 2 haploid sets of cabbage chromosomes (9 + 9) that can pair up normally during meiosis I.

The radish (Raphanus sativus) produces a crisp, edible taproot with many varieties, including white & red radishes, and giant oriental radishes 4 feet long and 40 pounds. The wild radish is a very common spring weed in San Diego County. Note: The bigeneric hybrid (Raphanobrassica) or rabbage is a cross between the radish (Raphanus n=9) and cabbage (Brassica n=9). The diploid hybrid has two sets of chromosomes, one set (R) from the radish parent and one set (C) from the cabbage parent. [Note: The word "set" is defined here as one haploid (n) set of chromosomes.] Since each set includes 9 chromosomes, the diploid rabbage has a total of 18 chromosomes. The diploid hybrid (RC) is sterile because the radish and cabbage sets of chromosomes are not completely homologous, and fail to pair up during synapsis of meiosis I. A fertile tetraploid (4n=36) hybrid (RRCC) has also been developed. It produces viable gametes and seeds because the radish chromosomes have another radish set to pair up with (RR), and the cabbage chromosomes have another set to pair up with (CC). Unfortunately this wonder plant has the leaves of the radish and the roots of the cabbage.

The massive taproot of wild radish (Raphanus sativus), a common naturalized weed in San Diego County, California. Unlike the numerous, tender, cultivated varieties of radishes, wild radish has a tough, woody taproot that is unpalatable


12. Polyploid Grains Of The Grass Family

A. Bread wheat (Triticum aestivum); B. Rye (Secale cereale); C. Triticale (Triticosecale). Wheat and rye are crossed together to produce the hybrid triticale.

Rye (Secale cereale) is a diploid plant (2n) composed of 2 sets of chromosomes (DD), each set with 7 chromosomes (D=7). [Note: The word "set" is defined here as one haploid set of chromosomes.] Therefore, the diploid number, or number of chromosomes in the rye sporophyte (DD), is 14. Bread wheat is a hexaploid (6n) composed of 6 sets of chromosomes (AA, BB & CC), each set with 7 chromosomes (A=7, B=7, C=7). Therefore, the number of chromosomes in the wheat hexaploid sporophyte (AABBCC) is 42. Triticale (Triticosecale) is a bigeneric hybrid between wheat (Triticum aestivum n=21) and rye (Secale cereale n=7). The resulting hybrid (ABCD) contains one set of rye chromosomes (D) and 3 sets of wheat chromosomes (ABC), a total of 28 chromosomes (7 + 21). It is sterile because the rye (D) set has no homologous set to pair up with during synapsis. This sterile hybrid seedling is treated with colchicine to produce a plant with twice as many chromosomes (i.e. 2A's, 2B's, 2C's and 2 D's), a total of 56. The fertile hybrid is an octoploid (8n) because it contains 8 sets of chromosomes. The diploid rye plant (DD) can also be crossed with tetraploid durum wheat (T. turgidum AABB) to produce a sterile triploid hybrid with 3 sets of chromosomes (ABD). This hybrid is treated with colchicine to produce a fertile hexaploid (6n) version of triticale (AABBDD). The following table shows a simplified summary of octoploid and hexaploid versions of triticale.

Durum wheat (Triticum turgidum ) is derived from wild emmer wheat of Syria. Emmer wheat is a tetraploid hybrid (4n=28) between einkorn wheat (T. monococcum or a relative) and a grass similar to the present-day goat grass (T. speltoides = Aegilops speltoides); or possibly T. longissima or T searsii. The original diploid (2n=14) emmer wheat was probably sterile because it contained only 2 sets of chromosomes, one from the einkorn parent (n=7) and one from the goat grass parent (n=7). Through a natural doubling of the chromosomes, a fertile tetraploid emmer wheat with 4 sets of chromosomes was produced. A mutation in the tetraploid emmer wheat, causing the bracts (glumes) enclosing the grain to break away readily, gave rise to the tetraploid durum wheat (T. turgidum or T. turgidum var. durum). The readily detachable grain makes the separation of the grain from the chaff relatively easy and is why durum wheat is called a "free-thrashing" type of wheat.

Tetraploid wheat also contains two proteins that combine to form a tenacious complex called gluten. Because of gluten, the wheat flour becomes elastic when mixed with water and kneaded, and when yeast is added, it rises into firm loaves. Yeast cells in the dough undergo fermentation and release carbon dioxide which becomes trapped in the glutinous protein mass. Baking "sets" the dough by drying the starch and denaturing the gluten protein. As the dough bakes, the carbon dioxide gas expands into larger bubbles, thus producing the porous, spongy texture of bread. Corn does not make good loaves of bread because it lacks gliadin, one of the key proteins of gluten. Consequently, corn bread crumbles and falls apart easily.

Two slices of bread made from different grains: Corn bread (left) and whole wheat (right). Corn bread does not make a firm loaf because it lacks gliadin, one of the key proteins of gluten. Without the elasticity of gluten, corn bread crumbles and falls apart readily.

Bread wheat (T. aestivum) is also a free-thrashing type of wheat. It is a hexaploid (6n) hybrid, four sets from an emmer wheat parent and two additional sets from a wild, weedy species (T. tauschii = Aegilops squarrosa). The endosperm of this hybrid wheat is especially high in protein and surpasses other wheats for bread making.


13. The Tetraploid Easter Lily

The commonly cultivated Easter lily (Lilium longiflorum) is a tetraploid (4n) hybrid. The typical haploid number for the genus Lilium is 12 (n = 12). The tetraploid hybrid has larger blossoms.

The fragrant white Easter lily (Lilium longifolium) is originally from southern Japan and Taiwan. Spectacular tetraploid hybrids of this beautiful lily are grown in greenhouses throughout the world. The white Madonna lily of Europe, depicted in numerous Renaissance paintings of the Annunciation, is Lilium candidum.


14. Hybrid Clarkias In San Diego County

An interesting example of hybridization and speciation in San Diego County, California: A. Willow-Herb Clarkia (Clarkia epilobioides), B. Delicate Clarkia (C. delicata), and C. Elegant Clarkia (C. unguiculata). Clarkia delicata (B) is a fertile tetraploid (4n) hybrid between C. epilobioides (A) and C. unguiculata (C).

The diploid (2n) hybrid between these two species is sterile because the nine C. epilobioides chromosomes and the nine C. unguiculata chromosomes are not truly homologous; therefore, the chromosomes fail to pair up properly during synapsis of Meiosis I. A naturally-occurring, fertile, tetraploid hybrid with four sets of chromosomes (two from each parent) gave rise to a new breeding population of C. delicata, which is considered to be a separate species. The hybrid has 18 chromosomes in its gametes (egg and sperm) and 36 chromosomes in the cells of the sporophyte. The petals lack the long stalk (claw) of C. unguiculata and are somewhat intermediate between the two parents. By the way, clawed mammals are referred to as unguiculates (mammals with hoofs are called ungulates), but in botany the term claw refers to the slender stalk of a petal.


15. A Hybrid Brodiaea With Strap-Shaped Staminodes

According to T. F. Niehaus (personal communication, 2004), hybrids are encountered where the distribution of two Brodiaea species overlap. The hybrids show a range of intermediate characteristics between the parents. Under controlled greenhouse conditions, Niehaus was able to hybridize every combination between all species and obtain seed. In a few crosses a relatively high percentage of seed set was obtained, although seed set was low in most interspecific crosses. He was able to grow the hybrid seeds to flowering in 2-3 years. According to Niehaus (1971), the majority of hybrids have sterile pollen; however, they can propagate by growing numerous cormlets per plant. Hybrid swarms are a fairly common phenomenon in other genera of flowering plants, including oaks (Quercus), penstemons (Penstemon) and prickly pears (Opuntia).

Sterile & Fertile Hybrid Brodiaeas In San Marcos

Left: Typical Coastal BTK (Brodiaea terrestris ssp. kernensis) showing hooded staminodia and fertile, pollen-bearing anthers. Right: Smaller variant with strap-shaped staminodia and sterile anthers without pollen. This is a sterile hybrid between Coastal BTK and B. filifolia or B. orcuttii which also occur intermixed in the field. Both B. orcuttii and B. filifolia even appeared in a flower pot with corms of the transplanted hybrid.

See Hybrid Brodiaea on the Santa Rosa Plateau

This appears to be a hybrid between Coastal BTK (Brodiaea terrestris ssp. kernensis) and B. filifolia or B. orcuttii which occur nearby. The strap-shaped staminodes are not hooded as in typical coastal BTK. The flower was sterile with no mature pollen grains, giving credibility to the hybrid hypothesis. In his "A Biosystematic Study of the Genus Brodiaea (Amaryllidaceae)", Univ. of Calif. Publications in Botany Vol. 60 (1971), Niehaus reported a cross between B. terrestris and B. coronaria which had a 100 percent seed set. The hybrid progeny of this cross had flowers that were morphologically intermediate between those of the two parents. Pollen fertility of each hybrid offspring was obtained, and the majority were completely sterile. According to Lyman Benson (Plant Taxonomy, The Ronald Press, New York (1962): "If, for example, more than half the pollen grains are abortive, probably something is amiss with the meiotic process preceeding pollen-grain formation." There are several causes for hybrid sterility, including the incompatibility between parental chromosomes during meiosis resulting in failure to pair up properly during synapsis of prophase I.

Left: Fertile, pollen-bearing stamen of Coastal BTK (Brodiaea terrestris ssp. kernensis). Right: Possible hybrid between Coastal BTK and B. filifolia or B. orcuttii which occur nearby. The small, thin anther sacs of the hybrid were devoid of pollen grains.

Niehaus (1971) suggests that Brodiaea polyploids are alloploids (crosses between different species), but then suggests that autoploidy may also be present. In addition, Niehaus states that "no meiotic irregularities or quadrivalents were observed in any polyploids." Normally homologous chromosomes pair up in 2's (bivalent) during synapsis of meiosis I. If homologous chromosomes associate in 4's rather than 2's, this is called a quadrivalent. Quadrivalent chromosome arrangements result in gametes with twice as many chromosomes and polyploids with higher numbers of chromosomes. If brodiaeas do not form quadrivalents, then allopolyploids are formed by adding up the gametophyte chromosome numbers of the two parents. For example, 24 chromosomes from B. terrestris ssp. kernensis plus 12 chromosomes from B. filifolia would result in a polyploid hybrid sporophyte with 36 chromosomes.

Staminode Variation On The Santa Rosa Plateau
Brodiaea jolonensis In San Diego County?
Coastal BTK In A Field In San Marcos


Two Hypotheses For Origin Of San Marcos Sterile Hybrid

Hypothesis #1

One possible explanation for hybrid sterility may be synaptic failure at prophase I of meiosis. Niehaus (1971) studied several populations of "Brodiaea jolonensis" in southern California and published sporophyte chromosome numbers of 36. We are certain that these populations are Coastal BTK and not B. jolonensis. My tentative count for Coastal BTK in San Marcos is also 36. If these populations are hexaploid (6n = 36) with a base number of 6 (n = 6), a cross between a tetraploid (4n) BO (or BT) would result in a 5n (pentaploid) hybrid. BO gametes would carry 2 sets of BO chromosomes, while Coastal BTK gametes would carry 3 sets of BTK chromosomes. The resultant hybrid would have 5 sets of chromosomes (5n = 30). Since the hybrid is an odd polyploid, one of the BTK sets has no homologous set to pair up with during synapsis:

5 Hybrid Chromosome Sets: BO & BO   BTK & BTK   BTK (no homologue)

Of course, the above explanation depends on a base number of 6. Based on my own count, Coastal BTK must be higher than 30. I suppose the sporophyte number could be 42, but then Coastal BTK itself will be a sterile odd polyploid. With 7 sets of chromosomes, one set will not have a homologous set to pair up with during synapsis. If Coastal BTK is 8n, then hybrids with BO and BT would be 6n and potentially fertile.

Microsporogenesis in the San Marcos BTK showing the first division of a pollen mother cell (microsporocyte). Cytoplasmic division (cytokinesis) has not occurred yet. The two chromosome clusters (2 sets of chromosome doublets) contain at least 36 chromosomes, possibly more depending on how you count overlapping chromosomes. There are very small chromosomes that may be obscured by the larger ones. Brodiaea species are known to have base numbers of 6 and 8. [500 x]

Diploidization Of Old Polyploids

Summarized from: "Advances in the Study of Polyploidy Since Plant Speciation."
by D.E. Soltis, P.S. Soltis, and J.A. Tate. New Phytologist 161: 173-191 (2003).

Old genomes that are polyploid with respect to the base number and amount of genetic material, may function as diploids with respect to the level of gene expression and chromosomal characteristics. These "old polyploids" may have become "diploidized" by the loss, mutation or suppression of duplicate genes. Other causes for diploidization may include genomic rearrangements and transposons. This can drastically change the chromosome properties of a species. For example, an odd polyploid with a base number of six might have a sporophyte number of 5n = 30. This pentaploid would tend to be sterile because of an odd chromosome set at synapsis of prophase I. However, if this plant behaves as a diploid with 2n = 30, it would be fertile with two sets of 15 chromosomes during meiosis. It would simply have a diploid (sporophyte) number of 2n = 30 and a haploid (gametophyte) number of n = 15.

If Brodiaea species behave as diploids, then BO and BF each have n = 12 and 2n = 24, and BTK has n = 18 or a higher number. In fact, BTK could be 2n = 36, 38, 40, 42, 44, 46, 48, etc. and still be theoretically fertile with two sets of homologous chromosomes. A hybrid between BO or BF and BTK would be sterile because of the non-homologous pairs of chromosomes that differ in number. Hybridization may occur, but the resulting hybrid offspring grown from seed may by sterile without viable pollen. For example, a cross between BO and BTK could result in a hybrid with 12 BO chromosomes and 18 BTK chromosomes which would not match up during synapsis of prophase I. This second hypothesis appears to better explain the sterile hybrids we have observed in San Marcos.

According to Niehaus (1971), the lack of quadrivalents in meiosis plus the differences in size of individual mitotic chromosomes at different ploidy levels suggests that these are "old polyploids." That is, the tetraploids, hexaploids, and octoploids have been in existence long enough to change by translocations, deletions, and so on. These differences apparently are enough to ensure that during meiosis only bivalents will occur in the higher ploidy levels.


Hypothesis #2

With base numbers of 6 and 8 chromosomes, most species of Brodiaea are technically polyploids with multiple sets of chromosomes. For example, B. terrestris ssp. kernensis in Kern County is an octoploid with eight sets of chromosomes, and B. orcuttii is tetraploid with four sets of chromosomes. [B. jolonensis in Monterey County is diploid with two sets of chromosomes.] According to Niehaus (1971), the lack of quadrivalents in meiosis plus the differences in size of individual mitotic chromosomes at different ploidy levels suggests that these are "old polyploids." That is, the tetraploids, hexaploids, and octoploids have been in existence long enough to change by translocations, deletions, and so on. These differences apparently are enough to ensure that during meiosis only bivalents will occur in the higher ploidy levels. In bivalents, only two sets of maternal and paternal chromosomes associate during synapsis of meiosis, in contrast to quadrivalent where four sets of homologous chromosomes associate during meiosis.

If Brodiaea species behave as diploids, then BF and BO each have n = 12 and 2n = 24, and BTK has n = 18 or a higher number. In fact, BTK could be 2n = 36, 38, 40, 42, 44, 46, 48, etc. and still be theoretically fertile with two sets of homologous chromosomes. A hybrid between BF or BO and BTK would be sterile because of the non-homologous pairs of chromosomes that differ in number. Hybridization may occur, but the resulting hybrid offspring grown from seed may by sterile without viable pollen. For example, a cross between BF and BTK could result in a hybrid with 12 BF chromosomes and 18 BTK chromosomes which would not match up during synapsis of prophase I. This hypothesis is a plausible explanation for the sterile hybrids we have observed in San Marcos.


16. Hybrid Oaks In San Diego County

Natural interspecific hybridization in oaks (Quercus): A. California Black Oak (Q. kelloggii), a tall, deciduous tree; B. Interior Live Oak (Q. wislizenii var. frutescens), a large, evergreen shrub; C. Oracle Oak (Q. x morehus), a small, partly deciduous tree that retains numerous leaves during the winter months. The leaves of Q. x morehus (C) have the intermediate size and lobes of Q. kelloggii), and the marginal spines of Q. wislizenii. All three species occur in the mountains of San Diego County, although the hybrid (Q. x morehus) is rare.

Natural interspecific hybridization in oaks (Quercus): A. California Black Oak (Q. kelloggii), a tall, deciduous tree; B. Gander Oak (Q. x ganderi), a partly deciduous tree that retains numerous leaves during the winter months; C. Coast Live Oak (Q. agrifolia), a large evergreen tree. The leaves of Q. x ganderi (B) have the intermediate size and lobes of Q. kelloggii), and the cup-like shape (upside-down cup) and pubescent (fuzzy), light green underside like Q. agrifolia. All three species occur in the mountains of San Diego County, although the hybrid (Q. x ganderi) is extremely rare.

See More Photos Of Oaks In San Diego County


17. Hybrid Pines In San Diego County

Natural interspecific hybridization in pines (Pinus): A. Coulter Pine (Pinus coulteri), with large seed cones composed of thick, hooklike scales; B. Coulter-Jeffrey Hybrid, with intermediate-sized cones composed of thick scales without conspicuous hooks; C. Jeffrey Pine (P. jeffreyi), with slightly smaller cones bearing scales with slender, downwardly-pointed prickles. In general growth aspect the Coulter-Jeffrey hybrid (B) resembles a Jeffrey pine. The bark even has the faint vanilla odor typical of Jeffrey pines.

The fragrance of Jeffrey pine bark is caused by aromatic aldehydes in the oleoresin. The fragrant aldehydes and alkanes (n-heptane) of Jeffrey pine oleoresins are absent in the turpentines of the closely-related ponderosa pine (P. ponderosa). On a warm day, when the bark is heated by the sun, the fragrant aldehydes are unmistakable. The seed cones of the hybrid are clearly intermediate between the parental species. The scales are thick and heavy as in the Coulter pine, but do not have the conspicuous hooks. The scales terminate in stout, outwardly-pointed (slightly hooked) prickles, unlike the slender, downwardly-pointed prickles of Jeffrey pine. In color, the hybrid cones are intermediate between the yellowish-brown cones of Coulter pine and the darker, reddish-brown cones of Jeffrey pine. Some hybrid cones show a closer resemblance to one or the other parent, probably due to backcrossing between the hybrid and one of the original parents.

The Coulter pine is actually more closely related to the Torrey Pine (P. torreyana) and digger pine (P. sabiniana) of coastal and central California. In fact, the latter three species are placed in the Group Macrocarpae, characterized by distinctive, large, heavy seed cones with thick cone scales and large seeds. The Jeffrey pine belongs to the Group Australes, along with the ponderosa pine and several other New World species. What is interesting about the Coulter-Jeffrey hybrid is that the parental trees belong to different genetic groups within the genus Pinus in which cross pollination between groups is uncommon. Coulter-Jeffrey hybrids are rare, but can be found occasionally in the Laguna Mountains of San Diego County. Because of the combined genetic traits of hybrid trees (called hybrid vigor), Coulter-Jeffrey hybrids are being planted and tested for resistance to drought and dwarf mistletoe (Arceuthobium campylopodum) in San Diego County.

References

  1. Armstrong, W.P. 1978. "Four Wildflowers Vanishing From Northern San Diego County." Environment Southwest Number 480: 3-6.

  2. Armstrong, W.P. 1977. "Natural Plant Hybrids in San Diego County." Environment Southwest Number 477: 14-16.

  3. Mirov, N.T. 1967. The Genus Pinus. Ronald Press Company, New York.

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