Does breeding ecology alter selection on developmental and life history traits? A case study in two Ambystomatid salamanders

Abstract

Terrestrial breeding with extended incubation has evolved repeatedly in fishes and amphibians but raises challenges for embryos, which must conserve sufficient yolk to continue development until water is available. One means of avoiding premature yolk consumption is for embryos to slow their development such that yolk is consumed more slowly. However slower development is associated with smaller adult body sizes and lower survivorship, particularly in amphibians occupying ephemeral habitats. The variable breeding strategies of Ambystomatid salamanders provide an excellent system in which to examine whether variation in breeding ecology is associated with divergence in embryonic development. Phylogenies of Ambystoma strongly support aquatic egg-laying and winter breeding as ancestral. Early breeding has evolved in three species and terrestrial breeding in two of the three. We propose that early, terrestrial breeding has altered selection on embryonic development. In particular, we propose that the prolonged incubation typically associated with early, terrestrial breeding has selected for longer development times. We compared embryonic development of the terrestrial breeding Ambystoma opacum (Gravenhorst 1807) with the co-occuring, aquatic breeding Ambystoma maculatum (Shaw 1802) under a range of laboratory conditions over two breeding seasons. We found that A. opacum embryos took longer to develop and hatched at a later stage, consistent with the hypothesis that early, terrestrial breeding favors embryos that extend development.