Encyclopedia of Evolution.pdf - Online Reading Center

0 hybridization
Three species of mustard (Brassica carinata, B. juncea, B. napus) have
arisen as hybrids between B. oleracea, B. nigra, and B. rapa. Boxes
represent original species; arrows represent hybrid species.
• The sunflowers Helianthus deserticola and H. anomalus
have been shown to have hybrid origins. Botanist Loren
Rieseberg has experimentally replicated the hybrid origin
of H. anomalus in the greenhouse. By crossing H. annuus
with H. petiolaris, Rieseberg produced three independent
hybrid lines. Only four generations were required to select
vigorous hybrids. The genetic markers (see quantitative
trait loci) of the greenhouse hybrids closely resemble
those in the wild populations of H. anomalus.
In plants, intergeneric hybrids (while still rare) are more
common than in animals. Russian botanist Georgi Karpechenko
crossed the radish Raphanus sativus with the mustard
Brassica oleracea, producing a Raphanobrassica hybrid,
in 1927. Unfortunately, the hybrid had the leaves of radish
and the roots of cabbage. Wild Raphanobrassica hybrids
have been reported from California. Triticale, a hybrid of
wheat and rye, is reported to have the high yield of wheat
and the hardiness of rye but is rarely grown on a commercial
scale. Hybrids between plant genera are known from
orchids and cypresses as well (see table on page 203). Among
orchids, hybrids are possible among three or even more genera,
although this almost always occurs under artificial conditions.
In natural habitats, pollinator specificity usually
prevents extensive crossings among orchids. For example,
Brassolaeliocattleya is a cross among the genera Brassavola,
Laelia, and Cattleya. When a scientific name is assigned to an
intergeneric hybrid, an × may precede the generic name.
Natural hybrids tend to be rare, but hybrid plants
produced for horticultural purposes can be vigorous. The
reduced reproductive capacity of the horticultural hybrid is
not a drawback because artificial propagation is available.
Perhaps the plant hybridizations that are most famous in
the history of the world are the crosses that produced modern
wheat. The wild wheat Triticum boeoticum had chromosomes
in pairs (diploid, 14 chromosomes) and was bred by ancient
farmers into the cultivated einkorn wheat Triticum monococcum.
However, T. boeoticum accidentally cross-pollinated
with the wild goatgrass Aegilops speltoides. The chromosomes
from the two parents were incompatible, but chromosome
doubling produced a fertile hybrid with chromosomes
in groups of four (tetraploid, with 28 chromosomes), the wild
T. dicoccoides. This was bred by ancient farmers into the
emmer wheat T. dicoccum and into durum (T. durum) and
other wheats. T. dicoccum also accidentally cross-pollinated
with a wild goatgrass, this time A. squarrosa. Chromosome
doubling turned a sterile hybrid into a species of wheat with
chromosomes in groups of six (hexaploid, 42 chromosomes),
which are today’s major wheat species T. spelta (spelt) and
T. aestivum (bread wheat) (see table on page 203). One of
the distinguishing features of modern bread wheat is the gluten
protein, which makes the flour sticky, allowing it to hold
in bubbles of carbon dioxide during leavening. The gene for
gluten apparently came not from the Triticum ancestor but
from the Aegilops squarrosa weed with which emmer wheat
accidentally hybridized.
Hybridization, aside from producing new plant species,
can also facilitate the transfer of genes from one species
of plant to another. Consider two plant species that
hybridize and produce a new hybrid species. The two original
plant species seldom hybridize with one another, but
may be able to crossbreed more often with the intermediate
hybrid species. Genes from the first plant species can enter
into the hybrid population by cross-pollination and can
then enter into the population of the second plant species,
also by cross-pollination. The hybrid species has thus acted
as a bridge over which genes have crossed, or introgressed,
from one species into another. This concept of introgressive
hybridization or introgression was first suggested by Edgar
Anderson, an early 20th-century expert on the genetic history
of crop species.
The relative rarity of hybridization between species suggests
that a new species, once it evolves, has a coordinated
team of genes that would be disrupted by mixing with a
different set of genes. This may explain why evolutionary
changes may occur so rapidly when a species first forms,
followed by a period of relative stability (see punctuated
equilibria).
Further Reading
Gompert, Zachariah, et al. “Homoploid hybrid speciation in an
extreme habitat.” Science 314 (2006): 1,923–1,925.
Schwarzbach, Andrea, Lisa A. Donovan, and Loren H. Rieseberg.
“Transgressive character expression in a hybrid sunflower species.”
American Journal of Botany 88 (2001): 270–277.