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. Hybrid Easter Lilies 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. Hybrid Brodiaeas Another interesting example of hybridization in San Diego County, California: A. Mesa Brodiaea (Brodiaea jolonensis), B. San Marcos Brodiaea (a rare hybrid with no scientific name) and C. Thread-Leaf Brodiaea (B. filifolia). The rare hybrid brodiaea appears to be a hexaploid (6n) hybrid between B. jolonensis (A) and B. filifolia (C). The San Marcos hybrid brodiaea has intermediate characteristics between the two parental species which grow in the same vacant field near Palomar College. [A fourth species (B. orcuttii) also grows in vernal pools on the property, but doesn't appear to be involved in this interspecific sexual relationship between brodiaeas. One of the most obvious characteristics that indicate hybridization is a structure called a staminodium which represents a sterile, nonfunctional stamen. All three species (A, B & C) have three, erect staminodia in the center of the flower, just outside the three yellow stamens. The three staminodia of the hybrid are intermediate between the conspicuous, petaloid staminodia of B. jolonensis (A) and the slender, inconspicuous staminodia of B. filifolia (C). The three staminodia of the hybrid appear strap-shaped and are visible in the center of the flower, just outside the three yellowish stamens. Because it is a polyploid (possibly a 6n hexaploid), the hybrid appears to be fertile and has formed a stable, reproducing population in a vacant field in the City of San Marcos near Palomar College. Unfortunately, the fate of this rare hybrid population is dismal because of the rapid (uncontrolled) urbanization of northern San Diego County. In this case, the rare brodiaea hybrid (and its rare parental species) just happen to be living in a habitat (a 40 acre field worth millions of dollars) surrounded by buildings and zoned for industry. Microsporogenesis in the rare San Marcos brodiaea hybrid showing the first division of the pollen mother cell into two cells, each containing about 18 chromosome doublets. Since Brodiaea species generally have six chromosomes per haploid set (n=6), this hybrid appears to have three sets of doublets in each cell after meiosis I, and three sets of single chromosomes in each cell following meiosis II. If the gametes contain three sets of chromosomes (3n), then cells of the adult sporophyte would contain six sets of chromosomes (6n). The only problem with this hypothesis is that the doubled chromosomes are difficult to count in the photo because of overlapping chromatids, so the exact number is only tentative at this time. Hybrid Oaks 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. Hybrid Pines 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 aldeydes 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. 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 meisis 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?

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