Asexual plant and animal lineages have long been considered evolutionary dead ends, incapable of generating significant genetic variability. Recent evidence, however, suggests that some asexual lineages may undergo subsexual processes that can lead to an increase in genetic variation, thereby improving evolutionary potential for adaptation in extreme environments. This dissertation research will examine whether recombination can be documented in an apomictic fern lineage. Apomictic ferns are capable of a unique type of sporogenesis (one not known to occur in flowering plants) where there is a possibility for homologous chromosomes to pair and recombine genetic information during meiosis. Using modern genomic approaches in combination with cytological studies, this study aims to identify whether such genetic variation exists in these ferns and the possible extent of this phenomenon in natural populations.
The proposed research will enhance current understanding of apomixis in ferns, an early diverging lineage of vascular plants that shares certain asexual traits with both animals and angiosperms. An increased knowledge of the biology of apomixis across plants is relevant not only for evolutionary and ecological studies of genetic variability in ferns, but can have wide-ranging implications for agricultural and crop sciences.
Genetic variation fuels evolutionary change. Without genetic variability—derived via mutation and spread by genetic mixing—organisms are profoundly susceptible to deleterious changes in their native environment. Thus, asexual organisms, being essentially clonal, are generally thought to be "evolutionary dead ends." However, some reproductive pathways may allow for genetic recombination to occur in these otherwise asexual lineages. In this study we investigated whether asexual (specifically, apomictic) plants are able to produce genetically variable offspring, in the absence of sexual reproduction. First, we developed genomic microsatellite markers (SSRs = simple sequence repeats); these were used to compare parent : offspring genotypes of the apomictic fern, Myriopteris lindheimeri. A total of 21 microsatellite markers were developed from data generated via the 454 GS-FLX Titanium sequencing platform. A total of 109 individuals collected from across the northern extent of the species range were genotyped for 8 of these markers to assess range-wide variation. We found that the percent polymorphic loci per population spanned 57.1–100% across the sites investigated. From those 109 plants, 41 highly heterozygous individuals were chosen as subjects for the parent : offspring genotype comparisons. A total of 847 offspring sporophytes (mean = 20 offspring per parent; ± 7.9, min = 5, max = 44) were genotyped for our 8 most variable SSR loci. We found that 27% of the parent plants produced genetically distinct offspring—in the absence of sex. Our results clearly indicate that this apomictic fern is capable of producing unique offspring via independent assortment and/or recombination. Evidence of this phenomenon is, in some cases, observed repeatedly across the genome, in individuals from across the species northern range. This is the first study to address the natural extent of this potentially widespread phenomenon, whereby otherwise asexual plants produce genetically variable offspring. Our findings indicate that this process may be a noteworthy factor influencing the evolution of apomictic ferns, as well as other similarly asexual/parthenogenetic organisms. We conclude that the dynamic nature of asexuality remains understudied and advocate continued exploration of related reproductive dynamics in plants, especially given widespread interest in the use of apomixis in agriculture.
