Mating system transitions impact population structure and biodiversity in Solanum habrochaites
Date
2018
Authors
Miller, Chris, author
Bedinger, Patricia, advisor
Ward, Sarah, committee member
Richards, Chris, committee member
Sloan, Dan, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Although many plant species have evolved means of preventing self-fertilization, self-compatibility (SC), the ability to set self-seed using pollen and ovules of the same plant is exceedingly common. In the wild tomatoes (Solanum section Lycopersicon) and other Solanaceous species, plants have evolved a genetic mechanism for preventing self-fertilization. Although many species are entirely either SC or self-incompatible (SI, unable to self-fertilize), one species of wild tomato, Solanum habrochaites, is notable for having separate SI and SC populations. In Chapter 1 of this thesis, I introduce mating system and self-incompatibility in Solanum habrochaites. Briefly, Solanaceous plants exhibit a specific type of gametophytic self-incompatibility controlled by cytotoxic, stylar-expressed S-RNases (and other factors) and pollen-expressed male resistance factors. At the northern species margin in southern and central Ecuador, S. habrochaites has undergone as least one SI → SC mating system transition. The loss of SI in this region coincides with a unique geographical feature, the Amotape Huancabamba Depression. In Chapter 2, I explore the loss of SI at the northern species margin using population genetics and reproductive biology. By analyzing the population structure of these populations in combination with controlled crosses, protein expression, and S-RNase allele screening, I identified at least four SC groups resulting from independent transitions from SI→SC. I also identified a fifth SC group of populations which likely arose due to the interbreeding of two separately derived SC populations. Stylar S-RNase protein expression can also be detected in this region, suggesting previously inactivated S-RNase genes in the parental groups may have become reactivated upon hybridization. In Chapter 3, I present the analysis of reproductive characters and morphology of newly collected populations of S. habrochaites. In one SC group, I find evidence of the "selfing syndrome," a phenomenon in which SC populations are predicted to possess small flowers and unexserted stigmas compared to their SI counterparts. This syndrome was not detected in the other SC populations, however. In Chapter 4, I describe two new "selfing" S-RNase alleles (hab-7, and hab-8) in different SC groups using degenerate primers and RNA-seq. hab-7 likely could encode a functional S-RNase protein, but it is likely unable to function in the SI response due to very low gene expression. The other allele, hab-8, detected in a different SC group, cannot express a functional S-RNase protein due to a single nucleotide substitution that produces a premature stop codon. Finally, in Chapter 5, I summarize my conclusions which support multiple SI→SC mating system transitions at the northern margin of S. habrochaites and review evidence that two distinct SC populations have interbred. I also suggest that some S-RNase alleles can be reversibly silenced and reactivated.