EVOLUTIONARY TRANSITIONS AMONG EGG-LAYING, LIVE-BEARING AND MATERNAL INPUTS IN SHARKS AND RAYS

Sharks and rays are thought to have a large number of independent orig ins of live-bearing. We examined evolutionary transitions to live-bear ing and maternal input to embryos in this subclass by optimizing repro ductive characters onto a composite phylogeny. Egg-laying (40% of all species) is the likely ancestral reproductive mode for this clade, and there is evidence that live-bearing has evolved independently 9-10 ti mes and maternal input 4-5 times. Most transitions (12-15) have been t oward live-bearing with provisioning limited to yolk. These have occur red from egg-laying ancestors or live-bearing taxa that provide matern al input to embryos. Only 2-3 transitions have occurred in the other d irection, i.e. away from yolk-only live-bearing. Egg-laying has evolve d from live-bearing ancestors in skates, Rajidae (25% of all species) and possibly in the zebra shark, Stegostoma fasciata. Thus, although t here has been an overall trend toward the evolution of live-bearing in elasmobranchs, the evolution of additional maternal input has been ex tremely labile.