Abstract
BackgroundPacu (Piaractus mesopotamicus) is one of the most important Neotropical aquaculture species from South America. Disease outbreaks caused by Aeromonas hydrophila infection have been considered significant contributors to the declining levels of pacu production. The current implementation of genomic selection for disease resistance has been adopted as a powerful strategy for improvement in fish species. This study aimed to investigate the genetic architecture of resistance to A. hydrophila in pacu via Genome-Wide Association Study (GWAS), the identification of suggestive Quantitative Trait Loci (QTLs) and putative genes associated with this trait. The genetic data were obtained from 381 juvenile individuals belonging to 14 full-sibling families. An experimental challenge was performed to gain access to the levels of genetic variation for resistance against the bacteria using the following trait definitions: binary test survival (TS) and time of death (TD).ResultsThe analyses of genetic parameters estimated moderate heritability (h2) for both resistance traits: 0.20 (± 0.09) for TS and 0.35 (± 0.15) for TD. A linkage map for pacu was developed to enable the GWAS, resulting in 27 linkage groups (LGs) with 17,453 mapped Single Nucleotide Polymorphisms (SNPs). The length of the LGs varied from 79.95 (LG14) to 137.01 (LG1) cM, with a total map length of 2755.60 cM. GWAS identified 22 putative QTLs associated to A. hydrophila resistance. They were distributed into 17 LGs, and were considered suggestive genomic regions explaining > 1% of the additive genetic variance (AGV) for the trait. Several candidate genes related to immune response were located close to the suggestive QTLs, such as tbk1, trim16, Il12rb2 and lyz2.ConclusionThis study describes the development of the first medium density linkage map for pacu, which will be used as a framework to study relevant traits to the production of this species. In addition, the resistance to A. hydrophila was found to be moderately heritable but with a polygenic architecture suggesting that genomic selection, instead of marker assisted selection, might be useful for efficiently improving resistance to one of the most problematic diseases that affects the South American aquaculture.
Highlights
Pacu (Piaractus mesopotamicus) is one of the most important Neotropical aquaculture species from South America
The cumulative mortality rate of the most susceptible family to A. hydrophila infection was 93.1%; in contrast, the most resistant family presented 40.7% of cumulative mortality rate, which indicated a considerable phenotypic variation associated to resistance against A. hydrophila infection (Fig. 1)
There have been several studies aiming to improve disease resistance via analysis of high-density Single Nucleotide Polymorphisms (SNPs) genotypes in aquaculture [13, 25, 26], but until the present date, no studies have focused on species produced in the Neotropical region
Summary
Pacu (Piaractus mesopotamicus) is one of the most important Neotropical aquaculture species from South America. GWAS identified 22 putative QTLs associated to A. hydrophila resistance They were distributed into 17 LGs, and were considered suggestive genomic regions explaining > 1% of the additive genetic variance (AGV) for the trait. While practically all Neotropical fish production is still carried out based on unselected stocks, large-scale genetic studies performed on global aquaculture species, such as carp [7,8,9], tilapia [10] and salmonids [11,12,13] have highlighted major possibilities to improve resistance to infectious diseases by selective breeding. Current advances in understanding the genetic architecture of resistance against A. hydrophila by GWAS-based conclusions have already been performed in some fish species, including carp Labeo rohita [20] and catfish hybrids between Ictalurus species [21] In both studies, several significantly associated QTLs to resistance against A. hydrophila were identified, containing candidate genes related to the immune responses
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