Within- and among-population impact of genetic erosion on adult fitness-related traits in the European tree frog Hyla arborea

Assessing in wild populations how fitness is impacted by inbreeding and genetic drift is a major goal for conservation biology. An approach to measure the detrimental effects of inbreeding on fitness is to estimate correlations between molecular variation and phenotypic performances within and among populations. Our study investigated the effect of individual multilocus heterozygosity on body size, body condition and reproductive investment of males (that is, chorus attendance) and females (that is, clutch mass and egg size) in both small fragmented and large non-fragmented populations of European tree frog (Hyla arborea). Because adult size and/or condition and reproductive investment are usually related, genetic erosion may have detrimental effects directly on reproductive investment, and also on individual body size and condition that in turn may affect reproductive investment. We confirmed that the reproductive investment was highly size-dependent for both sexes. Larger females invested more in offspring production, and larger males attended the chorus in the pond more often. Our results did not provide evidence for a decline in body size, condition and reproductive effort with decreased multilocus heterozygosity both within and among populations. We showed that the lack of heterozygosity–fitness correlations within populations probably resulted from low inbreeding levels (inferior to ca. 20% full-sib mating rate), even in the small fragmented populations. The detrimental effects of fixation load were either low in adults or hidden by environmental variation among populations. These findings will be useful to design specific management actions to improve population persistence.

评估野生种群中近交（inbreeding）与遗传漂变（genetic drift）对适合度（fitness）的影响，是保护生物学（conservation biology）的核心研究目标之一。检测近交对适合度产生不利影响的经典方法，是估算种群内及种群间分子变异与表型表现之间的相关性。本研究以欧洲雨蛙（Hyla arborea）的小型破碎化种群（fragmented populations）与大型非破碎化种群（non-fragmented populations）为研究对象，探究了个体多基因座杂合度（multilocus heterozygosity）对雄性与雌性的体型、身体状况及繁殖投入（reproductive investment）的影响：其中雄性的繁殖投入以鸣唱出勤（chorus attendance）为衡量指标，雌性则以产卵团质量（clutch mass）与卵大小为衡量指标。
鉴于成体体型和/或身体状况通常与繁殖投入存在关联，遗传侵蚀（genetic erosion）可能会直接对繁殖投入产生不利影响，也可能通过作用于个体体型与身体状况，进而间接影响繁殖投入。本研究证实，两性的繁殖投入均与体型高度相关：体型更大的雌性在后代生产上投入更多，体型更大的雄性则更频繁地前往池塘参与鸣唱聚集。
研究结果并未发现，无论在种群内还是种群间，多基因座杂合度降低会伴随体型、身体状况及繁殖投入的下降。我们进一步发现，种群内缺乏杂合度-适合度相关性，可能源于较低的近交水平（约低于20%的全同胞交配率（full-sib mating rate））——即便在小型破碎化种群中亦是如此。固定负荷（fixation load）的不利影响要么在成体阶段较为微弱，要么被种群间的环境差异所掩盖。
本研究结果可为制定针对性的种群存续管理措施提供科学参考。