Biology Reference
In-Depth Information
1989), a fact perhaps related to their success in colonizing northern areas
in postglacial time.
Homoploid hybrid speciation has been implicated in the origin of
species following invasion of new regions by ancestral species. One of the
most interesting examples of apparent homoploid hybrid speciation
involves sunflowers of the genus
Helianthus
. The common sunflower (
H.
annuus
) and the prairie sunflower (
H. petiolaris
), both annuals, were once
thought to have been restricted to largely nonoverlapping ranges in the
Great Plains (Heiser 1947).They are now sympatric over much of central
North America and occasionally interbreed, creating hybrid swarms con-
taining F
1
hybrids and individuals resulting from backcrosses.
More interestingly, Rieseberg (1991) concluded that three highly
localized species of
Helianthus
have resulted from hybridization of the
common and prairie sunflowers. Although these forms may not be good
examples of hybridization resulting from recent introductions of the
parental species to new regions, they clearly illustrate how homoploid
hybrid speciation can occur. These two parental species differ in at least
10 chromosomal translocations and inversions. In each of the derived
species, hybridization was followed by some reorganization of the
genome, leading to a form that is self-fertile, distinctive in morphology,
phytochemistry, and habitat, and strongly reproductively isolated from
both parents. Based on microsatellite differences, these daughter species
appeared to be at least 60,000 yr old (Rieseberg et al. 2003). Thus, they
may have arisen by hybridization during previous periods of climate
favorable to spread of the parent species into areas of the southwestern
United States. Two of the species,
H. anomalus
and
H. deserticola
,are
restricted to sand dune habitats in different locations. The third,
H. para-
doxus
, is restricted to brackish or saline marshes (Welch and Rieseberg
2002). For
H. anomalus
, Rieseberg et al. (1995) were able to map the pat-
tern of chromosomal reorganization in relation to the chromosome struc-
ture of the parent species. Hybridization involving introduced common
sunflowers and other native species of
Helianthus
has also resulted in the
origin of weedy subspecies or populations.
The genetic mechanisms that have enabled the puzzle sunflower
(
Helianthus paradoxus
) to succeed in a saline marsh environment unsuit-
able for either parent have been explored (Lexer et al. 2003a, 2003b).The
puzzle sunflower has from 5- to 14-fold greater salt tolerance than its par-
ents and exhibits more succulent leaves, as do many salt marsh plants.
When grown under irrigation by water of high salinity, puzzle sunflower
showed greater survivorship and maintained greater root biomass than its
