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 origins of live-bearing. We examined evolutionary transitions to live-bearing and maternal input to embryos in this subclass by optimizing reproductive 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 times and maternal input 4-5 times. Most transitions (12-15) have been toward live-bearing with provisioning limited to yolk. These have occurred from egg-laying ancestors or live-bearing taxa that provide maternal input to embryos. Only 2-3 transitions have occurred in the other direction, i.e. away from yolk-only live-bearing. Egg-laying has evolved from live-bearing ancestors in skates, Rajidae (25% of all species) and possibly in the zebra shark, Stegostoma fasciata. Thus, although there has been an overall trend toward the evolution of live-bearing in elasmobranchs, the evolution of additional maternal input has been extremely labile.