CRISPR knockouts of pmela and pmelb engineered a golden tilapia by regulating relative pigment cell abundance

Genetic basis and expression of ventral colour in polymorphic common lizards

Colour is an important visual cue that can correlate with sex, behaviour, life history or ecological strategies, and has evolved divergently and convergently across animal lineages. Its genetic basis in non-model organisms is rarely known, but such information is vital for determining the drivers and mechanisms of colour evolution. Leveraging genetic admixture in a rare contact zone between oviparous and viviparous common lizards (Zootoca vivipara), we show that females (N = 558) of the two otherwise morphologically indistinguishable reproductive modes differ in their ventral colouration (from pale to vibrant yellow) and intensity of melanic patterning. We find no association between female colouration and reproductive investment, and no evidence for selection on colour. Using a combination of genetic mapping and transcriptomic evidence, we identified two candidate genes associated with ventral colour differentiation, DGAT2 and PMEL. These are genes known to be involved in carotenoid metabolism and melanin synthesis respectively. Ventral melanic spots were associated with two genomic regions, including a SNP close to protein tyrosine phosphatase (PTP) genes. Using genome re-sequencing data, our results show that fixed coding mutations in the candidate genes cannot account for differences in colouration. Taken together, our findings show that the evolution of ventral colouration and its associations across common lizard lineages is variable. A potential genetic mechanism explaining the flexibility of ventral colouration may be that colouration in common lizards, but also across squamates, is predominantly driven by regulatory genetic variation.

Tilapia, a good model for studying reproductive endocrinology

The Nile tilapia (Oreochromis niloticus), with a system of XX/XY sex determination, is a worldwide farmed fish with a shorter sexual maturation time than that of most cultured fish. Tilapia show a spawning cycle of approximately 14 days and can be artificially propagated in the laboratory all year round to obtain genetically all female (XX) and all male (XY) fry. Its genome sequence has been opened, and a perfect gene editing platform has been established. With a moderate body size, it is convenient for taking enough blood to measure hormone level. In recent years, using tilapia as animal model, we have confirmed that estrogen is crucial for female development because 1) mutation of star2, cyp17a1 or cyp19a1a (encoding aromatase, the key enzyme for estrogen synthesis) results in sex reversal (SR) due to estrogen deficiency in XX tilapia, while mutation of star1, cyp11a1, cyp17a2, cyp19a1b or cyp11c1 affects fertility due to abnormal androgen, cortisol and DHP levels in XY tilapia; 2) when the estrogen receptors (esr2a/esr2b) are mutated, the sex is reversed from female to male, while when the androgen receptors are mutated, the sex cannot be reversed; 3) the differentiated ovary can be transdifferentiated into functional testis by inhibition of estrogen synthesis, and the differentiated testis can be transdifferentiated into ovary by simultaneous addition of exogenous estrogen and androgen synthase inhibitor; 4) loss of male pathway genes amhy, dmrt1, gsdf causes SR with upregulation of cyp19a1a in XY tilapia. Disruption of estrogen synthesis rescues the male to female SR of amhy and gsdf but not dmrt1 mutants; 5) mutation of female pathway genes foxl2 and sf-1 causes SR with downregulation of cyp19a1a in XX tilapia; 6) the germ cell SR of foxl3 mutants fails to be rescued by estrogen treatment, indicating that estrogen determines female germ cell fate through foxl3. This review also summarized the effects of deficiency of other steroid hormones, such as androgen, DHP and cortisol, on fish reproduction. Overall, these studies demonstrate that tilapia is an excellent animal model for studying reproductive endocrinology of fish.

Genome editing in East African cichlids and tilapias: state-of-the-art and future directions

African cichlid fishes of the Cichlidae family are a group of teleosts important for aquaculture and research. A thriving research community is particularly interested in the cichlid radiations of the East African Great Lakes. One key goal is to pinpoint genetic variation underlying phenotypic diversification, but the lack of genetic tools has precluded thorough dissection of the genetic basis of relevant traits in cichlids. Genome editing technologies are well established in teleost models like zebrafish and medaka. However, this is not the case for emerging model organisms, such as East African cichlids, where these technologies remain inaccessible to most laboratories, due in part to limited exchange of knowledge and expertise. The Cichlid Science 2022 meeting (Cambridge, UK) hosted for the first time a Genome Editing Workshop, where the community discussed recent advances in genome editing, with an emphasis on CRISPR/Cas9 technologies. Based on the workshop findings and discussions, in this review we define the state-of-the-art of cichlid genome editing, share resources and protocols, and propose new possible avenues to further expand the cichlid genome editing toolkit.

Downregulation of VEGF in the retinal pigment epithelium followed by choriocapillaris atrophy after NaIO3 treatment in mice

Sodium iodate (NaIO3) induces retinal pigment epithelium (RPE) dysfunction, which leads to photoreceptor degeneration. Previously, we used electron microscopy to show that the administration of NaIO3 resulted in the accumulation of cell debris in the subretinal space, which was thought to be caused by failed phagocytosis in the outer segment of the photoreceptor due to RPE dysfunction. We further analyzed the pathological changes in the retina and choroid of NaIO3-injected mice, and found that the expression of OTX2, an RPE marker, disappeared from central part of the RPE 1 day after NaIO3 administration. Furthermore, fenestrated capillaries (choriocapillaris, CC) adjacent to the RPE could not be identified only 2 days after NaIO3 administration. An examination of the expression of the CC-specific protein plasmalemma vesicle-associated protein (PLVAP), in sections and flat-mount retina/choroid specimens showed destruction of the CC, and complete disappearance of the PLVAP signal 7 days after NaIO3 administration. In contrast, CD31 flat-mount immunohistochemistry of the retina indicated no difference in retinal vessels between NaIO3-treated mice and controls. Electron microscopy showed that the fenestrated capillaries in the kidney and duodenum were morphologically indistinguishable between control and NaIO3-treated mice. We examined cytokine production in the retina and RPE, and found that the Vegfa transcript level in the RPE decreased starting 1 day after NaIO3 administration. Taken together, these observations show that NaIO3 reduces the CC in the early stages of the pathology, which is accompanied by a rapid decrease in Vegfa expression in the RPE.

Distinct genetic origins of eumelanin intensity and barring patterns in cichlid fishes

Pigment patterns are incredibly diverse across vertebrates and are shaped by multiple selective pressures from predator avoidance to mate choice. A common pattern across fishes, but for which we know little about the underlying mechanisms, is repeated melanic vertical bars. In order to understand genetic factors that modify the level or pattern of vertical barring, we generated a genetic cross of 322 F2 hybrids between two cichlid species with distinct barring patterns, Aulonocara koningsi and Metriaclima mbenjii. We identify 48 significant quantitative trait loci that underlie a series of seven phenotypes related to the relative pigmentation intensity, and four traits related to patterning of the vertical bars. We find that genomic regions that generate variation in the level of eumelanin produced are largely independent of those that control the spacing of vertical bars. Candidate genes within these intervals include novel genes and those newly-associated with vertical bars, which could affect melanophore survival, fate decisions, pigment biosynthesis, and pigment distribution. Together, this work provides insights into the regulation of pigment diversity, with direct implications for an animal's fitness and the speciation process.

Effects of Natural and Synthetic Astaxanthin on Growth, Body Color, and Transcriptome and Metabolome Profiles in the Leopard Coral Grouper (Plectropomus leopardus)

Natural and synthetic astaxanthin can promote pigmentation in fish. In this study, the effects of dietary astaxanthin on growth and pigmentation were evaluated in leopard coral grouper (Plectropomus leopardus). Fish were assigned to three groups: 0% astaxanthin (C), 0.02% natural astaxanthin (HP), and 0.02% synthetic astaxanthin (AS). Brightness (L*) was not influenced by astaxanthin. However, redness (a*) and yellowness (b*) were significantly higher for fish fed astaxanthin-containing diets than fish fed control diets and were significantly higher in the HP group than in the AS group. In a transcriptome analysis, 466, 33, and 32 differentially expressed genes (DEGs) were identified between C and HP, C and AS, and AS and HP, including various pigmentation-related genes. DEGs were enriched for carotenoid deposition and other pathways related to skin color. A metabolome analysis revealed 377, 249, and 179 differential metabolites (DMs) between C and HP, C and AS, and AS and HP, respectively. In conclusion, natural astaxanthin has a better coloration effect on P. leopardus, which is more suitable as a red colorant in aquaculture. These results improve our understanding of the effects of natural and synthetic astaxanthin on red color formation in fish.