What makes an egg unique? Clues from evolutionary scenarios of egg-specific genes

The avian egg, which contains the egg yolk, the egg white, and the eggshell, represents the mostly advanced amniotic egg in oviparous vertebrates. In mammals, this reproductive strategy of laying egg has gradually evolved toward placentation. In order to better understand the unique status of the avian egg in the evolution of the vertebrate reproduction, we investigated the evolution of some Gallus gallus egg-specific protein-coding genes. Based on our finding and other recent research, we have summarized here that gene formation (such as ovalbumin genes, ovocalyxin-36 and apovitellenin-1 encoding genes in the G. gallus), gene divergence between G. gallus and mammals (such as the ovocalyxin-32 gene with its ortholog, the mammalian RARRES1, and the ovocleidin-116 with its ortholog, the mammalian MEPE), and gene loss (egg-expressed genes lost during the evolution of the mammals, such as vitellogenin and RBP encoding genes) play significant roles in the evolution of egg-specific genes.