An ancient truncated duplication of the anti-Müllerian hormone receptor type 2 gene is a potential conserved master sex determinant in the Pangasiidae catfish family.

The evolution of sex determination (SD) in teleosts is amazingly dynamic, as reflected by the variety of different master sex-determining genes identified. Pangasiids are economically important catfishes in South Asian countries, but little is known about their SD system. Here, we generated novel genomic resources for 12 Pangasiids and characterized their SD system. Based on a Pangasianodon hypophthalmus chromosome-scale genome assembly, we identified an anti-Müllerian hormone receptor type Ⅱ gene (amhr2) duplication, which was further characterized as being sex-linked in males and expressed only in testes. These results point to a Y chromosome male-specific duplication (amhr2by) of the autosomal amhr2a. Sequence annotation revealed that the P. hypophthalmus Amhr2by is truncated in its N-terminal domain, lacking the cysteine-rich extracellular part of the receptor that is crucial for ligand binding, suggesting a potential route for its neofunctionalization. Reference-guided assembly of 11 additional Pangasiids, along with sex-linkage studies, revealed that this truncated amhr2by duplication is a male-specific conserved gene in Pangasiids. Reconstructions of the amhr2 phylogeny suggested that amhr2by arose from an ancient duplication/insertion event at the root of the Siluroidei radiation that is dated to ~100 million years ago. Together these results bring multiple lines of evidence supporting that amhr2by is an ancient and conserved master sex-determining gene in Pangasiids, a finding that highlights the recurrent use of the transforming growth factor ß pathway, which is often used for the recruitment of teleost master SD genes, and provides another empirical case towards firther understanding of dynamics of SD systems.

Adaptations of semen characteristics and sperm motility to harsh salinity: Extreme situations encountered by the euryhaline tilapia Sarotherodon melanotheron heudelotii (Dumeril, 1859).

In most teleost fishes, sperm cells are quiescent in the seminal plasma and are activated by either a drop (fresh water fish) or an increase in osmolality (marine fish) when released in the water. It is most interesting to examine how the mechanisms of sperm motility activation can adapt to a broad range of salinities, as applies to some euryhaline species, and particularly to the tilapia Sarotherodon melanotheron heudelotii, which can reproduce at salinities from 0 up to 120 in the wild. Here, the gonado-somatic index, semen characteristics, and the osmotic and ionic requirements of sperm motility activation were compared in S. m. heudelotii reared in fresh water (FW), sea water (SW), or hypersaline water (HW; salinities of 0, 35, and 70, respectively). No salinity-dependent differences were found in gonado-somatic index or semen characteristics, except for an increase of seminal plasma osmolality with increasing salinity (from 318 to 349 mOsm kg(-1) in FW and HW fish, respectively). The osmolality range allowing the highest percentages of sperm activation broadened and shifted toward higher values with increasing fish ambient salinity (150-300, 300-800, and 500-1200 mOsm kg(-1), for FW, SW, and HW fish, respectively). Nevertheless, at the three fish rearing salinities, sperm could be activated in media that were hypotonic, isotonic, or hypertonic relative to the seminal plasma, at least when some calcium was present above a threshold concentration. The [Ca(2+)] required for the activation of S. m. heudelotii sperm is (1) higher in fish reared at a higher salinity (2) higher in hypertonic than that in hypotonic activation media, whatever the fish rearing salinity, and (3) higher in the presence of Na(+) or K(+), the negative effects of which increased with an increase in fish rearing salinity. The [Ca(2+)]/[Na(+)] ​ ratios allowing for maximal sperm motility in SW or HW fish are close to those observed in natural environments, either in sea or hypersaline waters. In comparison to most teleosts with external fertilization, the total duration of sperm motility in S. m. heudelotii was exceptionally long (>2 hours regardless the fish rearing salinities). The decrease in sperm activity with increasing time since activation did not result from limiting energy reserves, as the addition of calcium in the activation medium caused most spermatozoa to become motile again. The comparison of sperm characteristics of S. m. heudelotii acclimated from FW to SW or HW with those of fish maintained all lifelong at their native salinity showed that adaptive responses were completed within 2 months or less.

Plasticity of gene expression according to salinity in the testis of broodstock and F1 black-chinned tilapia, Sarotherodon melanotheron heudelotii.

The black-chinned tilapia Sarotherodon melanotheron heudelotii Rüppell 1852 (Teleostei, Cichlidae) displays remarkable acclimation capacities. When exposed to drastic changes of salinity, which can be the case in its natural habitat, it develops quick physiological responses and keeps reproducing. The present study focused on the physiological impact of salinity on male reproductive capacities, using gene expression as a proxy of acclimation process. Two series of experimental fish were investigated: the first one was composed of fish maintained in freshwater for several generations and newly acclimated to salinities of 35 and 70, whereas the second one consisted of the descendants of the latter born and were raised under their native salinity. Expression patterns of 43 candidate genes previously identified from the testes of wild males was investigated in the three salinities and two generations. Twenty of them showed significant expression differences between salinities, and their predicted function revealed that most of them are involved in the osmotic tolerance of sperm cells and/or in the maintenance of sperm motility. A high level of expression variation was evidenced, especially for fish maintained in freshwater. In spite of this, gene expression patterns allowed the differentiation between fish raised in freshwater and those maintained in hypersaline water in both generations. Altogether, the results presented here suggest that this high variability of expression is likely to ensure the reproductive success of this species under varying salinities.

Impact of environmental DDT concentrations on gill adaptation to increased salinity in the tilapia Sarotherodon melanotheron.

Estuaries of tropical developing countries suffering from severe droughts induced by climate change are habitats to fish, which face drastic salinity variations and the contact with pollutants. The Western Africa tilapia Sarotherodon melanotheron is highly resistant to hypersalinity, but the effect of human-released xenobiotics on its adaptation is barely known. Controlled experiments were conducted to observe S. melanotheron gill adaptation to abrupt salinity variations in the presence of waterborne DDT, at concentrations detected in their natural habitat. The gills appeared as an important site of DDT conversion to DDD and/or depuration. A 12-days DDT exposure resulted in decreased gill epithelium thickness at all salinities (from fresh- to hypersaline-water), and the structure of gills from freshwater fish was particularly altered, relative to controls. No unbalance in tilapia blood osmolality was observed following DDT exposure, which however caused a decrease in branchial Na(+)-K(+)-ATPase (NKA) activity. Gill cellular NKA expression was reduced in salt-water, together with the expression of the CFTR chloride channel in hypersaline water. Although S. melanotheron seems very resistant (especially in seawater) to short-term waterborne DDT contamination, the resulting alterations of the gill tissue, cells and enzymes might affect longer term respiration, toxicant depuration and/or osmoregulation in highly fluctuating salinities.