Most work on hybrid incompatibilities has focused on characterizing specific incompatibilities at candidate genes, such as those associated with mapped QTL distinguishing species (Ting et al., 1998; Presgraves et al., 2003; Lee et al., 2008; Tang and Presgraves, 2009; Moyle and Nakazato, 2010).
Interestingly, our results may provide an explanation for the "mentor pollen" effect, which has been exploited with some success to overcome self- and inter-specific incompatibility (for references see [1], [2]).
In contrast, sex-specific incompatibility induces locus-specific barriers even at unlinked marker loci.
Concordant sex-specific incompatibility (for example, greater impairment of male hybrids or longer map lengths in females) induces lower barriers (higher rates of introgression) than expected under multiplicativity, and discordant sex-specific incompatibility induces higher barriers.
A new finding is that barriers to introgression due to sex-specific incompatibility depart in general from multiplicativity.
We proposed a scaling of the backward migration rate (Equation 3) that accounts for sex-specific incompatibility selection and linkage and showed that the distribution of waiting times along a random lineage traced backward conforms well to the distribution determined by this scaling.
While the homeobox gene Ods induces sterility in male hybrids between D. simulans and D. mauritiana (Ting et al. 1998), the oncogene Xmrk contributes to tumor formation in backcross Xiphophorus hybrids of both sexes (Schartl 2008), suggesting that sex-specific incompatibility is common but not universal.
For example, Table 1 indicates that in the absence of linkage with a neutral marker (rf = rm = 1 2 ), a single factor transmitted through both sexes that induces non-sex-specific incompatibility (σf = σm = σ ≤ 1) either imposes no barrier to introgression at the marker or reduces it by a factor of σ/(2 − σ) [compare the "gene flow factor" (Equation F6) of Bengtsson 1985].
In addition, we expected that inter-specific incompatibilities would show consistent distortion among maps.
Inter-specific incompatibilities may also be inferred from segregation distortion, as genotypes that occur less often than expected may be incompatible [ 3, 14].
