Transcriptome expression profiles between diploid and triploid Pacific abalone (Haliotis discus hannai) juveniles in response to acute heat-stress and hypoxia treatments

Abstract

With an increasing interest for the use of triploids in abalone aquaculture, it is crucial to understand their physiological responses to environmental stress, particularly such as heat-stress and hypoxia, which are significant factors that cause adverse effects on the efficiency and capacity of farming practice in abalone production. However, nothing is known about gene expression of triploid abalone to modulate physiological responses under different environmental stresses. Transcriptomic response to the acute heat-stress and hypoxia were explored in hepatopancreas of diploid and triploid Pacific abalone (Haliotis discus hannai) juveniles. A total of 316 million clean reads were de novo assembled into 271,039 contigs, of which a transcriptome with 209,974 non-redundant transcripts was produced. Using generalized fold change (GFOLD) algorithm with a cut-off │GFOLD value│ > 4, we identified differentially expressed transcripts (DETs) from diploid and triploid abalone in responses to acute heat-stress and hypoxia treatments, respectively. Comparative analysis of the identified DETs revealed alteration of transcript expression profile, level, and process in triploid abalone compared to their diploid siblings. Thus, our study will provide not only comprehensive insight into understanding of the transcriptional regulation to environmental stresses in triploid abalone but a framework for efficient management of triploid abalone aquaculture.

Introduction

Triploid animals possess three sets of homologous chromosomes in their somatic cells instead of the usual two in diploid organisms. This leads to either reproductive development or sterility according to the general assumption that the odd number of homologous chromosomes may not effectively divide during meiosis (Okumura et al., 2007; Piferrer et al., 2009). Within this context, triploidization has been considered as a confinement method to prevent unwanted reproduction of farmed aquatic animals. In addition, this technique has been considered as a potential means to improve quantitative traits of farmed animals due to the presumed ability of triploids to divert more metabolic flux to somatic growth achieved from reducing energy need for reproduction (Allen Jr and Downing, 1986; Liu et al., 2009).

The Pacific abalone (Haliotis discus hannai) is one of the most important marine gastropods and currently on the rise of annual aquaculture production in Korea and other East Asian countries (Cook, 2016; Park and Kim, 2013). Despite the significant increases in the production, abalone farming has recently faced some critical issues such as poor growth and high mortality, which have been often associated with hypoxia and high temperature of seawater during hot summer (Arai and Okumura, 2013; Shin et al., 2017). Previous studies on triploid mollusks indicated improvement of tolerance and survival to environmental stresses compared to their diploid counterparts (Allen Jr and Downing, 1986; Gagnaire et al., 2006; Shpigel et al., 1992). Moreover, although the physiological response of triploid abalone to environmental stress has not been extensively studied, a previous study has reported that certain genotype groups of triploid abalones (H. discus hannai) would exhibit significantly better survival compared to diploids when exposed to heat-stress (Fujino et al., 1987). These suggest that triploid abalone potentially display improved tolerance to heat-stress and possibly also to hypoxia. However, nothing is known about the difference between diploid and triploid abalones in their responses to heat-stress and/or hypoxia at molecular levels. Therefore, it is of interest to explore what gene expression patterns and molecular strategies triploid abalone use to respond and adapt to heat-stress and hypoxia, in comparison to their diploid counterparts.

Accordingly, we here designed the study to provide comprehensive transcriptome information of the triploid Pacific abalone (H. discus hannai) in response to acute heat-stress and hypoxia through the comparison with their diploid siblings. We employed paired-end NextSeq 500 sequencing technology generating six hepatopancreas RNA-Seq libraries, from which differentially expressed transcripts (DETs) in response to acute heat-stress and hypoxia were identified from the diploid and triploid H. discus hannai, respectively. Moreover, we compared the identified DETs between the diploid and the triploid abalone in differences in gene expression level and biological functions. Our transcriptome dataset provide a valuable genetic resource to gain deeper insights into the capability of triploid abalones to mount specific stress responses to heat-stress and hypoxia.