Meeting Abstract
Longstanding interest in the eco-evolutionary dynamics of insect-plant and host-parasite systems has yet to clarify how traits of either consumers or their obligate prey affect diversification rates. ‘Musical chairs’ models of host shifts versus ‘escape and radiate’ cycles of niche expansion have proven hard to test, and little work has examined coevolutionary dynamics in marine taxa. Sea slugs in clade Sacoglossa are host-specialized herbivores that repeatedly evolved photosynthetic abilities (kleptoplasty). We built a database of diet records for 420 species, and a molecular phylogeny for 282 ingroup taxa, to reconstruct their history of host use. Using comparative methods, we then assessed whether traits of slugs or algae (or particular host groups) were linked with increased diversification of slug lineages. Photosynthetic slugs had higher rates of host shifting and diversification, but the degree of host-association did not affect slug diversity. Slug lineages feeding on uncalcified algae diversified more, but host chemistry had no influence on consumer diversification. Ancestral reconstructions supported recurring, sequential transitions between four host groups: Halimedineae to Bryopsidineae to Cladophorales to Dascycladales/non-chlorophytes. Bryopsidineae feeding lowered diversification by 50% for non-photosynthetic slugs, but increased diversification by 50% for photosynthetic lineages, while Cladophorales feeders diversified at twice the rate of lineages on the ancestral host group. Shifts to more derived hosts occurred frequently but greatly decreased diversification. Transitions to uncalcified green algae in temperate zones thus opened new niches and spurred cladogenesis, especially for photosynthetic slugs, but transitions onto non-chlorophyte hosts yielded many evolutionary dead-ends.
