Is fertility of hybrids enough to conclude that the two oysters Crassostrea gigas and Crassostrea angulata are the same species?

Abstract

The distinction of the two cupped oysters Crassostrea gigas (Thunberg, 1793) and Crassostrea angulata (Lamark, 1819) into two species was chiefly due to their differing geographical distributions, C. gigas being present in Asia and C. angulata in Europe. Today it is commonly accepted that C. angulata and C. gigas are a single species according to morphological, genetic and F1 hybridization data. However, the demonstration of the fertility of their hybrids and the absence of any reproductive isolation remained to be investigated. Consequently, we studied the fertility of hybrids and sperm competition by performing three different experiments and producing G1 and G2 hybrid progenies between wild populations of C. angulata and C. gigas. Progenies showed very close developmental yields, at 24 hours after fertilization, according to dam taxa suggesting a strong maternal transmission of oocyte quality, but no reproductive isolation was observed between the two taxa. Significant decreases of developmental yields were noticed in C. angulata females with sperm competition, most probably due to early larval mortality. The fertility of hybrids C. angulata × C. gigas was demonstrated, which is further evidence that they are the same species. To definitively state the precise taxonomic classification of C. angulata and C. gigas, further studies are needed to (i) identify geographical zones where these taxa are in contact and (ii) assess their level of hybridization in these zones.

Artificial interspecific backcrosses between the hybrid of female Crassostrea hongkongensis × male C. gigas and the two parental species

“…Although a number of oyster breeders have conducted interspecific hybridization among the Crassostrea genus Batista et al, 2007Batista et al, , 2008Gaffney and Allen, 1993;Huo et al, 2013;Huvet et al, 2002Huvet et al, , 2004Piferrer et al, 2009;Xu et al, 2009Xu et al, , 2014Zhang et al, 2012Zhang et al, , 2014, there are no reports of further utilization of the hybrids with the exception of two subspecies of C. gigas and C. angulata (Wang et al, 2008). This is the first report to describe completely fertile backcrossed progeny and F 2 hybrids; this finding differed from the F 1 hybrids (C. hongkongensis ♀ × C. gigas ♂) that exhibited high levels of sterility .…”

Natural invertebrate hosts of iridoviruses (Iridoviridae)

“…4 Infection causing gill disease also reported from Crassostrea angulata (Lamarck) but C. gigas and C. angulata are now considered to be the same species based on morphological, genetic and progeny hybridization data (Huvet et al 2002). Characteristic gill disease was also reported in Mytilus edulis trossulus (Gould) from the Baltic Sea near Gdansk, Poland, but the causative agent was not identifi ed (Smolarz et al 2006).…”

Interspecific hybridization between Crassostrea angulata and C. ariakensis

“…Both Aa and aA crosses were able to generate individuals with mature gametes, and their sex ratios were similar to those of the pure species (Table 2). Similarly, Huvet et al (2002) found that progeny of C. gigas and C. angulata crosses were able to produce mature gametes.…”