Geostatistical analysis of flower color polymorphism reveals signatures of selection and breeding system.
Eckhart, Vincent1, Carter, Daniel2, McCord, Aleia1, 1 Grinnell College, Grinnell, IA, USA2 Scattergood Friends School, West Branch, IA, USA
ABSTRACT- Geostatistical analysis of genetic variation can detect spatial patterns that demand explanation and suggest hypotheses about ecological and evolutionary causes. We applied geostatistics to a polymorphism in flower color (petal spotting) in the bee-pollinated, California-endemic annual, Clarkia xantiana. Two subspecies of C. xantiana occur, one predominately outcrossing, the other highly self-pollinating. The subspecies replace each other along gradients in precipitation and pollinator abundance, both of which decline from west to east. We estimated color-morph frequency at two spatial scales: (1) among populations throughout the geographic range, with a total extent over 100 km; and (2) within natural populations of each subspecies, in 14 x 8 m grids. We analyzed frequencies by calculating semivariance as a function of lag distance, followed by kriging and mapping. The outcrossing subspecies showed little spatial dependence at either spatial scale, while the self-pollinating subspecies showed east-west clinal variation at the large scale and strong spatial dependence (3-5 m patches) within populations. Weak spatial dependence in the outcrossing subspecies at large scales is consistent with our other findings of balancing selection on petal spotting exerted by pollinators and herbivores. Weak spatial dependence within outcrossing populations may reflect substantial gene flow via pollen. In the self-pollinating subspecies, strong spatial dependence within populations may reflect reduced pollen dispersal that accompanies inbreeding, while the cline at the large scale suggests a geographic gradient in selection.
Key words: geographic variation, ecological genetics, geostatistics
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