Beschreibung:
This study compares the relationships among early development and life history characters between two monophyletic groups, salamanders and teleost fishes. Plethodontid salamanders have large eggs and slow development. Large egg size in plethodontid salamanders has been shown to influence several aspects of early development, including: (1) time of holoblastic cleavage, (2) thickness of the blastocoel roof, (3) gastrulation (morphogenetic processes and timing), (4) early developmental rate, (5) formation of an embryonic disk, and (6) percentage of egg volume contributing to embryonic structures. Egg size is just one of several factors that influence the rate of development. While the slow development of plethodontids may have evolutionary implications for timing of oviposition, the lack of a clear correlation between these variables indicates that other life history characters need to be studied. Comparisons of the timing of oviposition in 28 plethodontid species reveal that oviposition in the fall or winter is the derived condition. On the basis of six early developmental and six life history characters examined, there do not appear to be strong relationships between these two character sets. Evolutionary increases in egg size that delay when the egg cleaves holoblastically in some amphibian lineages (such as plethodontids) have been considered to be analogous to the changes that led to the evolution of meroblastic cleavage in such lineages as amniotes. However, teleosts provide an interesting contrast to this standard 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 of meroblastic cleavage may have significant relationships with life history traits because of osmotic influences and could qualify as a key innovation.