Sequence analysis of a fish vitellogenin cDNA with a large phosvitin domain

Vitellogenins (Vtg) are egg-yolk precursor proteins crucial for reproductive success in oviparous animals. We have cloned the first complete cichlid Vtg cDNA from the tilapia fish, Oreochromis aureus. This cDNA has the largest phosvitin (PV) domain amongst piscine Vtgs, being comparable to those of lamprey, Xenopus, and chicken. Thus, the size of PV is independent of the evolutionary advancement of a species. The closer interspecific relationship between O. aureus Vtg1 and Fundulus VtgII than the intraspecific relationship between Fundulus VtgI and II isoforms suggests that teleost ancestors had at least two Vtg isoforms. Contrary to the results of previous phylogenetic inference using Vtgs which indicate that insect lineage is most diverged and nematodes are closer to vertebrate lineage, our results show that nematodes and hexapods form two monophyletic sister groups. Another arthropod taxon, represented by a malacostracan crustacean, Penaeus japonicus, appears to be more closely related to the vertebrates than the hexapods.