Data from: Albinism in phylogenetically and geographically distinct populations of Astyanax cavefish arises through the same loss-of-function Oca2 allele

The Mexican tetra, Astyanax mexicanus, comprises 29 populations of cave-adapted fish distributed across a vast karst region in northeastern Mexico. These populations have a complex evolutionary history, having descended from "old" and "young" ancestral surface-dwelling stocks that invaded the region ~8 MYa and ~2.1 MYa, respectively. This study investigates a set of captive, pigmented Astyanax cavefish collected from the Micos cave locality in 1970, in which albinism appeared over the last two decades. We combined novel coloration analyses, coding sequence comparisons, and mRNA expression level studies to investigate the origin of albinism in captive-bred Micos cavefish. We discovered albino Micos cavefish harbor two copies of a loss-of-function Oca2 allele previously identified in the geographically distant Pachón cave population. This result suggests that phylogenetically young Micos cavefish and phylogenetically old Pachón fish inherited this Oca2 allele from the ancestral surface-dwelling taxon. This likely resulted from the presence of the loss-of-function Oca2 haplotype in the "young" ancestral surface-dwelling stock that colonized the Micos cave, and also introgressed into the ancient Pachón cave population. The appearance of albinism in captive Micos cavefish, caused by the same loss-of-function allele present in Pachón cavefish, implies that geographically and phylogenetically distinct cave populations can evolve the same troglomorphic phenotype from standing genetic variation present in the ancestral taxon.

墨西哥丽脂鲤（Astyanax mexicanus）包含29个洞穴适应性鱼类种群，这些种群分布于墨西哥东北部广袤的喀斯特地貌区域。这些种群拥有复杂的演化历史，其祖先分别为约800万年前（~8 MYa）和约210万年前（~2.1 MYa）入侵该区域的“古老”与“年轻”地表栖息种群。本研究针对1970年采集自米科斯洞穴（Micos cave）的圈养有色墨西哥丽脂鲤洞穴鱼种群展开探究，该种群近二十年来出现了白化症个体。本研究结合全新的体色分析、编码序列比对以及mRNA表达水平检测，探究圈养繁殖的米科斯洞穴鱼的白化症起源。研究发现，白化米科斯洞穴鱼携带两份功能丧失型Oca2等位基因，该等位基因此前在地理距离遥远的帕琼洞穴（Pachón cave）种群中被鉴定到。该结果表明，系统发育上较年轻的米科斯洞穴鱼与较为古老的帕琼洞穴鱼，均从祖先地表栖息类群中继承了该Oca2等位基因。这一现象很可能源于：入侵米科斯洞穴的“年轻”祖先地表栖息种群中存在该功能丧失型Oca2单倍型，且该单倍型还通过基因渐渗进入了古老的帕琼洞穴种群。圈养米科斯洞穴鱼的白化症由与帕琼洞穴鱼相同的功能丧失型等位基因导致，这意味着地理与系统发育背景均不同的洞穴种群，可通过祖先类群中存在的留存遗传变异，演化出相同的洞居退化表型（troglomorphic phenotype）。