Gonadal histology of adult western mosquitofish (Gambusia affinis) following early life exposure to 17α-ethynylestradiol, 17β- trenbolone, and/or atrazine

Confined animal feeding operations (CAFOs) are a source of endocrine disrupting compounds (EDCs) in the aquatic environment and introduced there through agricultural runoff and insufficient waste storage systems. This study used the three compounds 17α-ethynylestradiol (EE2), 17β- trenbolone (TB), and atrazine (Atr) alone and in a mixture representing a simulated CAFO runoff. Laboratory exposures were performed in order to assess if early life exposure during the period of sexual differentiation to CAFO runoff altered gonadal development in western mosquitofish (Gambusia affinis). Juveniles were exposed to low and high concentrations of EE2 (50 or 250 ng/L), TB (10 or 50 ng/L), Atr (150 or 1,500 ng/L), or simulated CAFO runoff (50 ng/L EE2, 10 ng/L TB, and 150 ng/L Atr or 250 ng/L EE2, 50 ng/L TB, and 1,500 Atr) for 30-days followed by a 46-day recovery period. Gonadal histology was measured in adult males and females to assess stages of sexual development (i.e., oogenesis or spermatogenesis) and gonadal histopathology. All of the ovaries analyzed were in the maturing or mature stages of oogenesis, and all of the testes examined were in the intermediate or advanced stages of spermatogenesis. Intersex was observed in one individual exposed to 1,500 ng/L Atr. Ovarian histopathology was normal in all females, however testicular histopathology showed a loss of cyst structure leading to unbound interstitial sperm cells and unsynchronized cyst development in all males except the control fish. This study provided some evidence that exposure to simulated CAFO runoff and its individual compounds impaired testicular development.

Chromosome-level genome assembly of the female western mosquitofish (Gambusia affinis)

Background The western mosquitofish (Gambusia affinis) is a sexually dimorphic poeciliid fish known for its worldwide biological invasion and therefore an important research model for studying invasion biology. This organism may also be used as a suitable model to explore sex chromosome evolution and reproductive development in terms of differentiation of ZW sex chromosomes, ovoviviparity, and specialization of reproductive organs. However, there is a lack of high-quality genomic data for the female G. affinis; hence, this study aimed to generate a chromosome-level genome assembly for it. Results The chromosome-level genome assembly was constructed using Oxford nanopore sequencing, BioNano, and Hi-C technology. G. affinis genomic DNA sequences containing 217 contigs with an N50 length of 12.9 Mb and 125 scaffolds with an N50 length of 26.5 Mb were obtained by Oxford nanopore and BioNano, respectively, and the 113 scaffolds (90.4% of scaffolds containing 97.9% nucleotide bases) were assembled into 24 chromosomes (pseudo-chromosomes) by Hi-C. The Z and W chromosomes of G. affinis were identified by comparative genomic analysis of female and male G. affinis, and the mechanism of differentiation of the Z and W chromosomes was explored. Combined with transcriptome data from 6 tissues, a total of 23,997 protein-coding genes were predicted and 23,737 (98.9%) genes were functionally annotated. Conclusions The high-quality female G. affinis reference genome provides a valuable omics resource for future studies of comparative genomics and functional genomics to explore the evolution of Z and W chromosomes and the reproductive developmental biology of G. affinis.