EVOLUTIONARY CORRELATIONS BETWEEN EARLY DEVELOPMENT AND LIFE-HISTORY IN PLETHODONTID SALAMANDERS AND TELEOST FISHES

This study compares the relationships among early development and life history characters between two monophyletic groups, salamanders and t eleost fishes. Plethodontid salamanders have large eggs and slow devel opment. Large egg size in plethodontid salamanders has been shown to i nfluence several aspects of early development, including: (1) time of holoblastic cleavage, (2) thickness of the blastocoel roof, (3) gastru lation (morphogenetic processes and timing), (4) early developmental r ate, (5) formation of an embryonic disk, and (6) percentage of egg vol ume contributing to embryonic structures. Egg size is just one of seve ral factors that influence the rate of development. While the slow dev elopment of plethodontids may have evolutionary implications for timin g of oviposition, the lack of a clear correlation between these variab les indicates that other life history characters need to be studied. C omparisons of the timing of oviposition in 28 plethodontid species rev eal that oviposition in the fall or winter is the derived condition. O n the basis of six early developmental and six life history characters examined, there do not appear to be strong relationships between thes e two character sets. Evolutionary increases in egg size that delay wh en the egg cleaves holoblastically in some amphibian lineages (such as plethodontids) have been considered to be analogous to the changes th at led to the evolution of meroblastic cleavage in such lineages as am niotes. However, teleosts provide an interesting contrast to this stan dard scenario: The evolution of meroblastic cleavage is not correlated with an increase in egg size, but rather, with a decrease in egg size . Changes in early development of teleosts that led to the evolution o f meroblastic cleavage may have significant relationships with life hi story traits because of osmotic influences and could qualify as a key innovation.