A remarkable pattern in the distributions of many sexual organisms and their obligately asexual counterparts, termed geographic parthenogenesis, is the predominance of asexual lineages in marginal environments characterized by unfavorable abiotic conditions and low biotic diversity. In several species of the red algal genus, Mastocarpus, inhabiting both Pacific and Atlantic oceans, sexual and asexual variants are largely sympatric, but asexual variants are more frequent at northern latitudes and sexual individuals are more frequent at southern latitudes. Despite the widespread occurrence of geographic parthenogenesis, and theory to explain it, in many marine, aquatic and terrestrial taxa, the ecological processes that generate and maintain these patterns remain relatively unaddressed.

     We are beginning to test several alternative hypotheses to explain geographic parthenogenesis in Mastocarpus spp. The shifting balance of life cycle types and lineages of Mastocarpus may emerge from differential reproductive output, differential production of new asexual lineages, differential viability of sexual and apomictic variants across a latitudinal gradient, and/or a historical pattern of differential colonization. This work is being initiated in collaboration with Drs. J. E. Kübler (CSU-Northridge), C. Maggs, and J. Provan (Queen's University of Belfast, UK). Our approach combines field and laboratory experiments across a latitudinal gradient to establish (1) ecological patterns of productivity, reproductive effort and survivorship in sexual and asexual variants and (2) phylogeographic patterns of genetic diversity and distance between, as well as a coalescent approach to gene genealogies of the two life history variants.

     By using permanently marked and mapped individuals of M. papillatus, we are developing tools (genetic markers, fluorescence microscopy of nuclear volume) to distinguish the isomorphic sexual and asexual gametophytes. Phylogeographic studies of M. papillatus of several different established markers are being used to infer whether sexual and asexual lineages have historically differed in biogeographic dispersal and perhaps to detect life cycle specific markers. We have established that in sympatry, the life history variants often show similar patterns on the smaller scale of within a shore as they do at large geographic scales across shores: asexual individuals are in more marginal environments higher on the shore (submitted).