Intraspecific genetic variation underlying postmating reproductive barriers between species in the wild tomato clade (Solanum sect. Lycopersicon)

A goal of speciation genetics is to understand how the genetic components
underlying interspecific reproductive barriers originate within species.
Unilateral incompatibility (UI) is a postmating prezygotic barrier in
which pollen rejection in the female reproductive tract (style) occurs in
only one direction of an interspecific cross. Natural variation in the
strength of UI has been observed among populations within species in the
wild tomato clade. In some cases, molecular loci underlying
self-incompatibility (SI) are associated with this variation in UI, but
the mechanistic connection between these intra- and inter-specific pollen
rejection behaviors is poorly understood in most instances. We generated
an F2 population between SI and SC genotypes of a single species, Solanum
pennellii, to examine the genetic basis of intraspecific variation in UI
against other species, and to determine whether loci underlying SI are
genetically associated with this variation. We found that F2 individuals
vary in the rate at which UI rejection occurs. One large effect QTL
detected for this trait co-localized with the SI-determining S-locus.
Moreover, individuals that expressed S-RNase—the S-locus protein involved
in SI pollen rejection—in their styles had much more rapid UI responses
compared to those without S-RNase protein. Our analysis shows that
intraspecific variation at mate choice loci—in this case at loci that
prevent self-fertilization—can contribute to variation in the expression
of interspecific isolation, including postmating prezygotic barriers.
Understanding the nature of such intraspecific variation can provide
insight into the accumulation of these barriers between diverging
lineages.