Genetic and phenotypic characterisation of commercial dusky kob (Argyrosomus japonicus) cohorts

[摘要] ENGLISH ABSTRACT: Dusky kob, Argyrosomus japonicus, is a large Sciaenid finfish that has been identified as an emergent aquaculture species in South Africa. Current production practices for dusky kob are based on mass spawning of genetically unimproved (wild-caught) broodstock. In recent years, considerable efforts have been initiated to retain first filial (F1)-generation animals with fast growth rate as potential broodstock for a selective breeding programme. Although a few studies have been conducted on the species, previous studies have not addressed fundamental questions related to the effects of mass spawning production practices for the development of a selective breeding programme for dusky kob. This study aimed not only to bridge this gap, but also to investigate, for the first time, the potential for selection for increased growth rate in dusky kob. By using 14 microsatellite markers, the genetic properties of a wild population (n = 34) were compared to an F1 cohort that represents three temporal groups that were sampled throughout the production cycle (i.e. from weaning to market size). Despite a heterozygote excess, likely as a result of a genetic bottleneck, the F1 cohort displayed comparatively low levels of allelic diversity with respect to the wild population (P < 0.01). This was attributed primarily to the establishment of a small founder breeding population (n = 12), but also to low participation amongst females during the spawning event. Parentage analyses indicated only five (full-sib) F1 families. Families with low starting contributions were not eliminated following removal of the smallest animals by culling. Culling, however, did contribute to a significant increase in genetic relatedness and a single family represented 88% of the market-sized group, suggesting that these practices may have the potential to further complicate the selection of unrelated broodstock in commercial mass spawning species. Pedigree relations were inferred for an additional three F1 cohorts each produced from a breeding population comprising no more than five wild captive broodstock. Averaged relatedness amongst the three F1 cohorts was comparatively higher than that detected for the F1 animals of the first spawning event analysed. Furthermore, estimates of direct heritability (h2) were 0.34±0.25 and 0.36±0.27 for juvenile weight and length, respectively, and the genetic correlation between the traits was 0.98±0.03. Although estimates of h2 are likely biased due to small sample sizes, and possibly maternal and/or competition effects, it is concluded that selective breeding for increased growth rate can be successful in juvenile dusky kob. However, the current analysis indicates that F1 broodstock candidates are likely to be related and when bred will lead to excessive inbreeding. As this could have grave consequences for the profitability of dusky kob production, it is advisable that a selection programme for the species will need to consider both individual growth performance and genetic relatedness, e.g. using walk-back selection. Continued monitoring is therefore advised.

[发布机构] Stellenbosch University