Autotetraploid Pacific oysters (Crassostrea gigas) obtained using normal diploid eggs: induction and impact on cytogenetic stability

Abstract

We describe two methods of producing viable and fertile autotetraploid Pacific oyster (Crassostrea gigas Thunberg) based on the use of normal-sized oocytes produced by normal diploid females. Our methods showed that the oocyte size is not a limiting factor for the success of the induction to autotetraploidy. These methods offer means of direct introgression of genetic progress from elite diploid lines to tetraploids used as broodstock, avoiding a triploid step with the risk of transferring undesirable traits from highly fecund triploids. High variability in the level of cytogenetic stability was found among the different tetraploid oysters tested, showing that induction method has an important impact on the long-term cytogenetic stability of the tetraploids. It appears that induction method based on the use of triploid females induces a greater cytogenetic instability among tetraploids so obtained, and this compared to tetraploids originating from the two methods described in our present study. As the aneuploidies and reversions observed in tetraploids can have serious consequences for the sustainability of tetraploid broodstock itself, as well as their triploid offspring, the two tetraploid induction methods described in the present work offer means to produce tetraploids with optimal cytogenetic, genetic, and zootechnical performances.

Aneuploid progeny of the American oyster, Crassostrea virginica, produced by tetraploid × diploid crosses: another example of chromosome instability in polyploid oysters

“…In a recent paper by Benabdelmouna and Ledu (2015), heritability of CIN was further implicated by observations from tetraploids derived through a completely different method of induction, the D r a f t so-called direct method -produced by inhibiting the first polar body in a 2n x 2n cross (also see below). Below, we discuss the observations of mosaicism from the parents in this study, our observations of CIN in the crosses made from these parents, and insights that these observations and other work tell us about the heritability of CIN in mosaic tetraploid oysters.…”

“…Moreover, by definition, these types of tetraploids derive from "fertile" triploids (Guo andAllen 1994b, Eudeline et al 2000). Alternative methods to produce tetraploids exist (McCombie et al 2005b, Benabdelmouna andLedu 2015), but for all types, subsequent generations of tetraploids are propagated from tetraploid x tetraploid crosses (Guo and Allen 1997). Virtually, all commercial triploid oysters in the world are now created using the tetraploid x diploid cross.…”

“…Observations of chromosome clumping in mosaics were made on chemically induced triploids and mated (4n x 2n) triploids (Zhang et al 2010a) and chemically induced tetraploids (Zhang et al 2013). Other evidence for mosaics have been made through FCM, including chemical triploid C. gigas (Allen et al 1996), chemical triploid C. ariakensis (Zhou 2002), mated triploid C. ariakensis (Erskine 2003), mated tetraploid C. virginica (Matt and Allen 2014) and, now, on direct tetraploids of C. gigas (Benabdelmouna and Ledu 2015). It would seem, then, that the general condition of polyploidy provides the substrate for subsequent chromosome loss.…”

“…It is curious that there seems such a difference in the incidence of mosaicism in the several types of tetraploids documented by Benabdelmouna and Ledu (2015). They compared three types of tetraploids, which we will name after the authors responsible: GA -those induced from triploid eggs (Guo and Allen 1994a); MC -those produced by fertilizing diploid eggs with tetraploid sperm, followed by second polar body inhibition (McCombie et al 2005b); and, BLthose created directly by inhibiting the first polar body in a 2n x 2n cross (Guo et al 1992a;Benabdelmouna and Ledu 2015). In comparison of the three types of tetraploids at two years old, the proportion of mosaics was GA -45%, MC -25%, and BL -7%.…”

Genetic mapping in polyploids

“…between 50 and 100 million years ago (Allendorf et al, 2015). Polyploidy is also sometimes artificially induced in animals, for example in Pacific oysters (Crassostreae gigas) (Benabdelmouna and Ledu, 2015) or the silkmoth (Bombyx mori L.) (Rasmussen and Holm, 1979). There continues to be debate about whether any polyploid mammals exist, with the initial claim that the Argentinian red vizcacha rat (Tympanoctomys barrerae) is tetraploid (Gallardo et al, 1999) being more recently challenged in light of new data (Svartman et al, 2005;Evans et al, 2017).…”