Barker, Michael S. .
Asymmetrical Reproductive Isolation and Polyploid Species Establishment in Homosporous Ferns.
Homosporous ferns possess the largest average chromosome number (n=57.05) and estimated incidence of polyploid speciation (~7% of speciation events) among extant plant lineages. Given these outstanding features, relatively little is known about polyploid species establishment in homosporous ferns. To better understand polyploid species establishment, reproductive isolation among diploid and tetraploid cytotypes was examined in the laboratory using Ceratopteris richardii gametophytes. Autotetraploids of C. richardii were generated by growing leaf cuttings on agar and collecting the resulting diploid gametophytes. Reciprocal crosses were conducted between the spore grown haploid gametophytes and the leaf generated diploid gametophytes. To minimize potential genotypic interactions, intragametophytic self-fertilization was utilized to generate all parental plants. The preliminary results suggest that significant, asymmetrical reproductive isolation exists among the C. richardii cytotypes. Crosses within cytotypes (i.e., diploid × diploid & tetraploid × tetraploid) yielded 87% (diploid) and 72% (tetraploid) successful fertilization rates, as measured by sporophyte production. However, crosses among cytotypes showed a distorted ratio; diploid sperm × tetraploid eggs had a success rate of 58%, whereas tetraploid sperm × diploid eggs had a success rate of 3%. It is hypothesized that the putatively larger tetraploid sperm are occluded from the narrower diploid archegonia, whereas the putatively smaller diploid sperm have free access to the tetraploid archegonia. Further research is needed to confirm this hypothesis, and includes further crossing experiments in the lab to support the current trend and SEM data to confirm sperm size exclusion. If this hypothesis is supported by future work, the implications for polyploid species establishment are clear. In a mixed population, tetraploids may be out-fertilized by their diploid relatives whereas the diploid taxa will likely not suffer similar competition.
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1 - Indiana University, Department of Biology, Jordan Hall, 1001 East Third Street, Bloomington, Indiana, 47405, USA
Presentation Type: Oral Paper
Date: Monday, August 15th, 2005
Time: 10:30 AM