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.