In addition to the premating barriers created by adaptive divergence in mating system, flowering time, and abiotic requirements, speciation depends on the development of postzygotic barriers such as hybrid sterility. Current lab projects on speciation investigate the joint role of rearrangements as direct causes of hybrid sterility and suppressors of recombination, the mechanisms and origins of hybrid incompatibilities, and the causes of conspecific pollen precedence.
Chromosomal rearrangements, including inversions and translocations, may play dual roles in speciation. First, rearrangements can (and must, in the case of most translocations) cause F1 hybrid sterility, generating costs of heterozygosity that make their spread to define species paradoxical. Second, they suppress recombination in heterozygotes, and thus contribute to the packaging of locally adapted traits and other reproductive barriers in the face of gene flow. Current projects include studies of inversion polymorphism within Mimulus guttatus (e.g. Lee, Fishman et al. 2016) and work on rearrangements and reproductive barriers in the M. cardinalis complex (e.g., Fishman et al. 2013, Stathos & Fishman 2014).
Hybrid sterility and inviability
Hybrid inviability and sterility are important contributors to speciation, and also provide a window into the drivers and consequences of genomic divergence within lineages. We have worked on diverse hybrid incompatibilities, including interactions among nuclear loci, cytonuclear epistasis (Fishman & Willis 2005, Barr & Fishman 2010), chromosomal rearrangements (Stathos & Fishman 2014), and hybrid seed lethality (Garner et al 2016). Current work on hybrid incompatibilities is primarily focused on genomic effects of selfish mitochondrial evolution and nuclear coevolution (e.g., Case et al. 2016).
Transmission ratio distortion
Post-mating and post-zygotic reproductive barriers filter genetic transmission beyond the first generation of hybridization. Thus, patterns of transmission ratio distortion (TRD) in mapping populations can reveal both selfish genetic elements segregating within species and genomic incompatibilities that prevent gene flow, We have used TRD to study both hybrid incompatibilities (hybrid lethality, gametophytic male sterility) and conspecific pollen precedence (Fishman et al. 2008, Aagaard et al. 2013).