Evolutionary transitions among egg-laying, live-bearing and maternal inputs in sharks and rays
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| Authors: | Dulvy, NK; Reynolds, JD |
| Year: | 1997 |
| Journal: | Proceedings of the Royal Society of London Series B-Biological Sciences 264: 1309-1315 |
| Title: | Evolutionary transitions among egg-laying, live-bearing and maternal inputs in sharks and rays |
| Abstract: | 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. |
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