Data from: Evidence of the phenotypic expression of a lethal recessive allele under inbreeding in a wild population of conservation concern

Deleterious recessive alleles that are masked in outbred populations are predicted to be expressed in small, inbred populations, reducing both individual fitness and population viability. However, there are few definitive examples of phenotypic expression of lethal recessive alleles under inbreeding conditions in wild populations. Studies that demonstrate the action of such alleles, and infer their distribution and dynamics, are required to understand their potential impact on population viability and inform management responses. The Scottish population of red-billed choughs (Pyrrhocorax pyrrhocorax), which currently totals <60 breeding pairs and is of major conservation concern, has recently been affected by lethal blindness in nestlings. We used family data to show that the pattern of occurrence of blindness within and across affected families that produced blind nestlings was exactly 0·25, matching that expected given a single-locus autosomal lethal recessive allele. Furthermore, the observed distribution of blind nestlings within affected families did not differ from that expected given Mendelian inheritance of such an allele. Relatedness estimates showed that individuals from affected families were not more closely related to each other than they were to individuals from unaffected families that did not produce blind nestlings. Blind individuals tended to be less heterozygous than non-blind individuals, as expected if blindness was caused by the expression of a recessive allele under inbreeding. However, there was no difference in the variance in heterozygosity estimates, suggesting that some blind individuals were relatively outbred. These results suggest carriers of the blindness allele may be widely distributed across contemporary families rather than restricted to a single family lineage, implying that the allele has persisted across multiple generations. Blindness occurred at low frequency (affecting 1·6% of observed nestlings since 1981). However, affected families had larger initial brood sizes than unaffected families. Such high fecundity of carriers of a lethal recessive allele might reflect overdominance, potentially reducing purging and increasing allele persistence probability. We thereby demonstrate the phenotypic expression of a lethal recessive allele in a wild population of conservation concern, and provide a general framework for inferring allele distribution and persistence and informing management responses.

在远交种群中被掩盖的有害隐性等位基因（deleterious recessive alleles），预计会在小型近交种群（inbred populations）中得以表达，进而降低个体适合度（individual fitness）与种群存续能力（population viability）。然而，野生种群近交条件下致死隐性等位基因（lethal recessive alleles）表型表达的确切例证仍较为匮乏。为明确此类等位基因对种群存续能力的潜在影响，并为保护管理决策提供科学依据，亟需开展相关研究以阐明其作用机制，推导其分布与动态变化规律。当前仅存不足60个繁殖对、受到重点保护关注的苏格兰红嘴山鸦（Pyrrhocorax pyrrhocorax）种群，近期出现了雏鸟致死性失明的病例。本研究借助家系数据证实：在产生失明雏鸟的受影响家系内部及家系间，失明的发生比例恰好为0.25，与单基因常染色体致死隐性等位基因（autosomal lethal recessive allele）的理论预期模式完全一致。此外，受影响家系内失明雏鸟的实际分布，与该等位基因遵循孟德尔遗传（Mendelian inheritance）的理论预期并无显著差异。亲缘关系估算（relatedness estimates）结果显示，受影响家系的个体间亲缘系数并未高于其与未出现失明雏鸟的对照家系个体间的亲缘程度。失明个体的杂合度（heterozygosity）普遍低于非失明个体，这与近交条件下隐性等位基因表达引发失明的理论预期相符。但杂合度估算值的方差并无显著差异，提示部分失明个体实际处于相对远交的状态。上述结果表明，该失明等位基因的携带者可能广泛分布于当前多个家系中，而非局限于单一家系谱系，这暗示该等位基因已在种群中存续了多代。失明现象的发生频率较低：自1981年以来，观测到的雏鸟中仅有1.6%受到该病症影响。但受影响家系的初始窝卵数显著高于未受影响家系。致死隐性等位基因携带者所具备的这种高繁殖力，可能源于超显性（overdominance）效应，这或可减缓基因清除（purging）进程，并提升该等位基因的存续概率。本研究据此证实了受保护野生种群中致死隐性等位基因的表型表达，并为推导等位基因的分布与存续规律、制定针对性保护管理策略提供了通用研究框架。