PARENT SESSION
OOS 1: Genetic Explorations of the Seascape: Using Molecules and Experiments to Understand Marine Biodiversity.
Organized by: E Sotka and R Thacker
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room E 141 .

Genetic and geographic variation in a marine plant-herbivore interaction.

Sotka, Erik^*,1, ¹ Stanford University, Pacific Grove, CA

ABSTRACT- Marine consumers are often picky about the foods they eat, especially when their prey produce formidable arrays of chemical, nutritional and morphological deterrents. A window by which to explore the ecology and evolution of feeding preference and tolerance for prey defenses is afforded by the genetic and geographic variation that occurs within a single species of consumer. However, such variation is poorly described for marine organisms, either because few studies have addressed the issue or because the feeding preferences of marine populations rarely differentiate. I address this using an herbivorous amphipod Ampithoe longimana (Crustacea). Where the amphipod co-occurs with terpene-rich brown seaweeds in the tropical genus Dictyota, the amphipod readily consumes, and lives on Dictyota. On average, these sympatric populations have stronger feeding preference for Dictyota species and greater fitness when raised on Dictyota than do populations that are outside the geographic range of Dictyota. These results are consistent with the hypothesis that selection favors amphipods that tolerate Dictyota's chemical defenses and thereby access its enemy-free space. Amphipods allopatric with Dictyota are unable to tolerate the chemical defenses of Dictyota, either because of selection, drift or some mix of both forces. Phylogeographic analysis of mtDNA and nuclear sequences indicate a strong historical break between amphipod populations sympatric with Dictyota and populations more than 500 km beyond the geographic endpoint of Dictyota. The historical separation of these most northerly populations could have allowed a random accumulation of alleles to effectively degrade tolerance for Dictyota, although selection may also play a role in maintaining low Dictyota tolerance. These results indicate for the first time that populations of a marine herbivore respond evolutionarily to their seaweed hosts, raising the spectre that seaweed-herbivore interactions may coevolve.

Key words: seaweed, herbivore, secondary metabolite, geographic variation