Sex identification from distinctive gene expression patterns in Antarctic krill (Euphausia superba)

Abstract

Antarctic krill (Euphausia superba) is a highly abundant keystone species of the Southern Ocean ecosystem, directly connecting primary producers to high-trophic level predators. Sex ratios of krill vary remarkably between swarms and this phenomenon is poorly understood, as identification of krill sex relies on external morphological differences that appear late during development. Sex determination mechanisms in krill are unknown, but could include genetic, environmental or parasitic mechanisms. Similarly, virtually nothing is known about molecular sex differentiation. The krill genome has to date not been sequenced, and due to its enormous size and large amount of repetitive elements, it is currently not feasible to develop sex-specific DNA markers. To produce a reliable molecular marker for sex in krill and to investigate molecular sex differentiation we therefore focused on identifying sex-specific transcriptomic differences. Through transcriptomic analysis, we found large gene expression differences between testes and ovaries and identified three genes exclusively expressed in female whole krill from early juvenile stages onwards. The sex-specific expression of these three genes persisted through sexual regression, although our regressed samples originated from a krill aquarium and may differ from wild-regressed krill. Two slightly male-biased genes did not display sufficient expression differences to clearly differentiate sexes. Based on the expression of the three female-specific genes we developed a molecular test that for the first time allows the unambiguous sex determination of krill samples lacking external sex-specific features from juvenile stages onwards, including the sexually regressed krill we examined.

A thorough annotation of the krill transcriptome offers new insights for the study of physiological processes

“…We used the final assembly described above to re-estimate transcript abundances over a wide range of RNAseq dataset (see Table S3 ) including: Larval krill at two different stages of development exposed to different CO 2 conditions, coming from 17 (Table S3 , Group 1) Adult krill (48 samples) coming from different geographical areas (Bransfield Strait, Lazarev Sea, South Georgia, South Orkney) and different seasons (summer and winter), divided into male and female specimens 21 (Table S3 , Group 2) Adult krill exposed to three different temperatures—Low Temperature, Mid temperature, High Temperature (Table S3 , Group 3) Adult krill divided into male and female specimens 50 (Table S3 , Group 4) …”

The most comprehensive annotation of the Krill transcriptome provides new insights for the study of physiological processes and environmental adaptation

“…We used the final assembly described above to re-estimate transcript abundances over a wide range of RNAseq dataset (see Table S1 ) including: Larval krill at two different stages of development exposed to different CO 2 conditions, coming from [17] ( Table S1 – Group 1) Adult krill (48 samples) coming from different geographical areas (Bransfield Strait, Lazarev Sea, South Georgia, South Orkney) and different seasons (summer and winter), divided into male and female specimens [35] ( Table S1 – Group 2) Adult krill exposed to three different temperatures – Low Temperature, Mid temperature, High Temperature ( Table S1 – Group 3) Adult krill divided into male and female specimens [36] ( Table S1 – Group 4) …”

“…• Adult krill exposed to three different temperatures -Low Temperature, Mid temperature, High Temperature (Table S1 -Group 3) • Adult krill divided into male and female specimens [36] (Table S1 -Group 4) Overall, these datasets include six experimental factors: geographical area, season, developmental stage, pCO2 exposure condition, sex and temperature. Newly computed transcript abundances and raw counts were imported using R (version 4.0.5) and the package tximport (version 1.18.0).…”

Redclaw Crayfish Cherax quadricarinatus LRPPRC : The characterization of a sex‐linked gene and its relationship with vitellogenin

“…To map the sex‐determining region or markers accurately, a large number of markers are needed (Yu et al, 2017). To produce a reliable molecular marker for sex in krill Euphausia superba and to investigate their molecular sex differentiation, Suter et al (2019) found large differences in gene expression between testes and ovaries and identified three female‐specific genes using transcriptomic analysis. These three female transcripts, vitellogenin receptor ( VGR ), nuclear autoantigenic sperm protein ( NASP ) and polehole‐like protein ( PHL ), allow for an unambiguous molecular test to determine the sex of krill samples.…”