Hybridization and reproduction isolation
Under laboratory conditions, mating between C. sonorensis andC. occidentalis can produce viable offspring for at least six
generations, though the hatch rate of the progeny is dependent on the
species of the mother (Velten & Mullens, 1997). A cross of femaleC. sonorensis and male C. occidentalis only yields a 7%
hatch rate whereas the reciprocal cross yields a 75% hatch rate. This
asymmetrical hybrid viability is likely caused by cytonuclear
incompatibility (Arntzen, Jehle, Bardakci, Burke, & Wallis, 2009;
Gibeaux et al., 2018), though endosymbionts have also been shown to
cause reproductive incompatibility (Werren, Baldo, & Clark, 2008). Upon
secondary contact of closely related species, and in the absence of
post-zygotic reproductive isolation, the production of unfit hybrids can
induce the rapid evolution of premating barriers (Coyne & Orr, 2004;
Howard, 1993; Servedio & Kirkpatrick, 1997; Yukilevich, 2012). In most
populations however, C. sonorensis females are unlikely to come
across C. occidentalis males due to differences in mating
behavior. Conversely, C. occidentalis females do come into
contact with C. sonorensis males, who do not appear to have mate
discrimination (Downes, 1978), and will likely attempt to mate with
these heterospecific females. As there are demographic disparities
(population size and structure) between these two species, as well as
viable offspring produced from this cross, rampant hybridization and
asymmetric introgression would be detrimental to C. occidentalis(Toews & Brelsford, 2012). Strong selection against hybridization can
maintain species boundaries, but as two of the ten C.
occidentalis collected from Borax Lake in California (CABL) were F1
hybrids, another mechanism, potentially ecological or behavioral
isolation, appears to be limiting directional introgression fromC. sonorensis .
Culicoides occidentalis females lay their eggs in highly
saline environments (up to 88.0 parts per thousand (ppt)) (Smith &
Mullens, 2003), and C. sonorensis eggs will not hatch in water
with salinity over 20.0 ppt (Linley, 1986). Ecological exclusion via the
larval habitat is present in this system, but alone would only limit
introgression if the survival rate of the hybrids were reduced.Culicoides occidentalis mate at the larval habitat while C.
sonorensis mates at or near a host (Gerry & Mullens, 1998; Holbrook et
al., 2000), and this difference in mating behavior may be a more likely
mechanism by which the detrimental effects of hybridization are
diminished. Most C. occidentalis females will mate at the larval
habitat, but if this does not happen, they may be mated by C.
sonorensis while feeding at the host. As C. occidentalis females
return to the high saline pools to lay their eggs, these hybrid
individuals would have an increased chance of backcrossing with theC. occidentalis lineage. While only two C. occidentalis xC. sonorensis hybrids were tested in this study, both hadC. occidentalis mothers, providing some evidence that this is the
scenario taking place in nature.