Nature and timing of dysregulation associated with pseudoalbinism in California Halibut, Paralichthys californicus

Pseudoalbinism in flatfishes, the absence of melanophores or melanin over most of the ocular side of the body, is often observed in hatcheries. The prevalence of pseudoalbinism can be very high, sometimes exceeding 90% of individuals in California Halibut, Paralichthys californicus . Pseudoalbinism is not observable prior to metamorphosis, but its prevalence is influenced by rearing conditions during the larval period. This project used quantitative real time polymerase chain reaction (qRT‐PCR) methods to measure the relative expression of eight focal genes (sox10a, sox10b, gch2, cyp26b1, aldh1a2, aldh1a3, asip1 and LOC109629521 ) in California Halibut at four developmental stages (stages F, G, H and I). Batches of halibut with a high prevalence of pseudoalbinism (94%, 97%) and intermediate prevalence (42%, 42%) were sampled. For reference, Southern Flounder (Paralichthys lethostigma ) with a low prevalence (1%, 2%) were also sampled. Strong differences in gene expression were observed at stage G, particularly gch2, asip1, cyp26b1 and sox10b . Viewing all stages together and including the low‐prevalence data for Southern Flounder revealed a sharp decrease (>10‐fold) in gch2 expression in the high prevalence group of halibut but little change in the low prevalence flounder. gch2 expression for the intermediate group became bimodal, suggesting that the final pigmentation phenotype became fixed during a narrow developmental window when halibut were 7.2–7.6 mm standard length. The observed patterns of gene expression suggest a hypothesis that pseudoalbinism in California Halibut arises at the transition from melanoblasts to mature melanophores during mid‐ to late metamorphosis (stages G and H) possibly as a result of antagonism of the melanocortin‐1 receptor (MC1R) and ectopic expression of the normal blind‐side developmental pathway. Expression of gch2 may be a candidate marker for screening for incipient pseudoalbinism, pending formal validation.