Colloquia: Systematic and Evolutionary Perspectives on Apomixis
Campbell, Christopher , Burgess, Michael B. , Frye, Christopher , Cushman, Kevin R. , Doucette, Eric .
The species problem in agamic complexes: An approach in Amelanchier (Rosaceae).
In agamic complexes diploids are mostly sexual and polyploids often apomictic. Diploid speciation is usually slow and facilitated by barriers to reproduction with other lineages. Within sexual species, gene flow is potentially cohesive. In contrast, speciation coincides with the formation of polyploids, which are reproductively isolated and can persist as coherent units through apomixis. Many sexual species contain considerable morphological and genetic diversity, whereas apomicts readily form genetically and morphologically relatively invariant groups. To delimit species, we favor de Queiroz’s general lineage species concept because it calls upon multiple criteria, not all of which must be met. We use the criteria of morphological distinctness, reproductive isolation, and allelic coalescence in Amelanchier, in which the diploids that have been studied are sexual and the polyploids are apomictic. The diploid species we have adequately sampled are well-marked lineages, at least partly meeting all three of our criteria. For polyploids we use the additional ranking criterion of the size of the geographic range. Polyploid groups that are morphologically distinct but locally distributed are microspecies, and we do not recognize them with scientific names because they can confound the taxonomy of agamic complexes by resembling diploid species morphologically, evolving recurrently, and being numerous. Microspecies that expand to at least a regional geographic distribution are accepted as species. Because Amelanchier polyploids are pollen-fertile and facultatively apomictic, they can diversify through sexuality and hybridization. We infer that, over time, these polyploid species become more heterogeneous, their boundaries become fuzzy, and they transform into species complexes. Microspecies, polyploid species, and species complexes form a continuum from morphologically distinct and locally distributed to bewilderingly complex and widely distributed. Understanding the evolution of polyploid taxa in agamic complexes informs a usable species classification, which for Amelanchier polyploids reserves scientific names for taxa that are morphologically discrete and geographically widespread.
The species problem is challenging in agamic complexes, groups wherein reproduction is at least partly by apomixis (asexual seed production) and hybridization is common. Assembly of data from morphology, DNA sequences, ploidy level, and field observations in Amelanchier is expected to increase our understanding of diversification and the formation of taxa in this agamic complex. This understanding should buttress a usable classification of the species in this genus and possibly enhance taxonomy in other agamic complexes.
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1 - University of Maine, School of Biology & Ecology, Orono, ME, 04469-5735, USA
2 - Maryland Department of Natural Resources, Natural Heritage Program, 909 Wye Mills Road, Wye Mills, MD, 21679, USA
3 - University of Maine, School of Biology & Ecology, Orono, ME, 04469-5735
Presentation Type: Symposium or Colloquium Presentation
Location: 551A/Convention Center
Date: Wednesday, August 4th, 2010
Time: 4:30 PM