Mutation, Selection and Divergence of Plant Populations

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Evolutionary biology seeks to understand the processes underlying the diversity of life on earth. All current diversity-whether among species, populations or individuals-originated by mutation. Mutations accumulating in geographically separated populations can generate both phenotypic divergence and reproductive isolation, the first step in the formation of new species. The research proposed here will address how mutations cause population divergence and reproductive isolation within Lobelia cardinalis, a widespread diploid species ranging from southern Canada to Central America. Because of multiple paternity, the male and female functions of plants have different optimal levels of offspring provisioning. Female parents are related by half to each of their offspring. Male parents, in contrast, are related by half only to their own offspring and by less than half to offspring of other sires. This difference generates a sexual conflict: in theory, female parents benefit from equal provisioning of all offspring, while each male parent benefits by higher provisioning of its own offspring at the expense of other males. We have (1) discovered that crosses between geographically divergent L. cardinalis populations often result in abnormally small or large seeds and developed methods (2) to test for evidence of sexual conflict and (3) to distinguish sexual conflict from other causes of abnormal offspring provisioning. This set of projects will investigate both the genomics and developmental biology of reproductive isolation arising from sexual conflict evolving independently in geographically separated populations. We have recently discovered that experimental crosses among geographically separate populations of L. cardinalis always produce seeds but that, depending on the population pair and the crossing direction, the seeds are either all sexual, all asexual (apomictic) or a mixture. Sexual seed production in angiosperms involves a complex set of coordinated interactions between maternal and paternal genomes in both embryo and endosperm. Apomixis following pollination represents complete post-mating reproductive isolation. Using molecular and microscopy techniques developed in our lab, we will explore the genomic, genetic and developmental basis of apomixis in L. cardinalis, including the role of repetitive elements and genome size. This set of projects will thus address the evolutionary origin of both apomixis and post-mating reproductive isolation, a defining step in speciation. The proposed research aims to advance our understanding of the combined roles of mutation and selection in shaping plant evolution and the divergence of populations. The projects provide modern and advanced training opportunities at both the graduate and undergraduate levels in several areas including bioinformatics, computer-based simulation, confocal microscopy, statistical analysis and field biology.