青背山雀(Parus monticolus)幼鳥個體的異型合子歧異度與幼鳥體型及免疫力的關係

Abstract

異型合子優勢假說(heterozygosity advantage hypothesis)認為異型合子 (heterozygous)個體比同型合子個體有較高的適存性(fitness)。因此，天擇傾向最大化個體的異型合子歧異度。為驗證異型合子優勢假說，已有大量的研究針對生存能力、配對行為、繁殖成功率及疾病的抵抗能力等適存性特徵與個體的異型合子歧異度相關性進行探討，但大多數的研究都以成體的適存性特徵為研究主題。本實驗則於2007及08年3月至7月中旬以台灣中部青背山雀(Parus monticolus) 幼鳥為研究題材，以18組微衛星體(microsatellite)基因座估算個體的異型合子歧異度(以內在近親指數internal relatedness，IR表示)，以複迴歸控制巢及巢中雛數效應後，檢測幼鳥的異型合子歧異度與幼鳥孵化後的第3、11日齡體重、跗趾長及免疫力的相關性。結果顯示幼鳥內在近親指數與幼鳥免疫力(p=0.010) 呈現顯著的負相關，且Titgata89基因座中異型合子個體的第3日齡體重卻比同型合子個體輕(p = 0.0001)。此研究清楚的呈現異型合子歧異度與幼鳥適存性相關之形態特徵及生理特徵間的相關性，並推測可能受整體效應及局部效應的影響。另外，異型合子歧異度與幼鳥形態特徵間相關的現象，隨著幼鳥發展階段改變，在幼鳥體重發育趨於穩定時，異型合子歧異度與形態特徵的相關性消失，此項結果可為前人研究無法發現個體的異型合子 歧異度與成體形態特徵間相關性提供可能的解釋。

Heterozygosity advantage refers heterozygous individual having higher fitness than homozygous ones, i.e. natural selection may favor individuals with maximum genetic heterozygosity. Association between neutral genetic heterozygosity of an individual and its fitness components, such as survival and fecundity, are well established, but the relationship between heterozygosity and morphological traits of nestlings has been little studied. Here, I measured 99 green-backed tit (Parus monticolus) nestlings’ body mass and tarsus length on the 3rd and 11th day since hatching, and immune response. I then regressed these factors on individual heterozygosity index: individual’s internal relatedness (IR), as estimated from 18 microsatellite loci. The result obtained by multiple linear regression after controlling for nest and brood size shows significant negative correlation between immunocompetence and IR (p=0.010) , and positive local effect of IR at locus Titgata89 on body mass on the 3rd day of hatching (p=0.0001). This study clearly reveals heterozygosity fitness correlations in terms of both general effect and local effect. Furthermore, I found that correlations between heterozygosity and body mass during early developmental period disappear in later period; this may provide explanation for the lack of correlation between adult heterozygosity and morphological traits.