What Makes an Egg Unique? Clues from Evolutionary Scenarios of Egg-Specific Genes1

Abstract

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.

Ovocalyxin-36 and other LBP/BPI/PLUNC-like proteins as molecular actors of the mechanisms of the avian egg natural defences

“…Both phylogeny and genomic comparison suggest that the chicken TENP gene is orthologous to the mammalian BPIL1 ( Figure 2). Probably, we observe a case of functional divergence after duplication event between TENP and BPIL1 genes [26]. The BPIL2 protein was also detected in egg white and in vitelline membranes [37,38].…”

“…The sequence comparison reveals that OCX-36 is related to the antibacterial protein superfamily BPI/LBP/PLUNC with 20-27 % identities between OCX-36 and BPI/LBP/PLUNC members [BPIL3, LPLUNC3, LPLUNC4 and LPLUNC6) [16,20,26]. The similarity for conservative substitutions ranges from 39 to 44 % and the Blast E-values between OCX-36 and these proteins indicated a significant relationship.…”

“…is nested in a tandemly arranged BPI/LBP/PLUNC gene cluster on the chromosome 20, and it is in particular next to BPIL3, LPLUNC3, LPLUNC4 (Figure 2) [26] and LPLUNC6 [20]. Synteny analysis reveals that the BPI/LBP/PLUNC gene cluster is conserved in amniotes (including both birds and mammals), except that OCX-36 is specifically present in the chicken genome but not in any mammalian genomes.…”

“…The exon/intron organization of the OCX-36 gene [16] is very similar to that of the highly conserved LBP and BPI genes [28], with most corresponding exons possessing identical sizes ( Figure 3). Recently, evolutionary scenarios of egg-specific genes were established to evaluate the significance of gene formation, gene divergence and gene loss that are associated with the avian egg [26]. OCX-36 protein has originated by specific gene formation in birds.…”

Dynamics of Structural Barriers and Innate Immune Components during Incubation of the Avian Egg: Critical Interplay between Autonomous Embryonic Development and Maternal Anticipation

“…More than 700 proteins have been identified in this organic matrix [93, 95]. Some are also associated with other egg structures (OVAL, LYZ, and TF), while others are relatively ES-specific (ovocleidin [OC]17 and OC116; ovocalyxin [OCX]21, OCX25, OCX32, and OCX36) and involved in the biomineralization of the ES [92, 96]. The ES protein osteopontin has recently been shown to control ES nanostructure, where the dimensions of the calcite mineral structure correlate with ES hardness [97].…”