遺傳合適性假說提出：雌性應與最適合的雄性交配來產下擁有最適合基因型的子代。如果適應性是與所有基因中的異型合子有關，那麼這個假說就預測雌性應會與遺傳上最不相似的雄性交配以生下擁有較多異型合子的子代。自2002至2004年，我們在中台灣針對社會性一夫一妻制的青背山雀來測試遺傳合適性假說，總共用了9個多變異的微衛星體基因座來計算青背山雀基因中的異型合子及配對間的遺傳相似度。發現沒有偶外交配行為的配對(NEPF PAIRS)(平均Rxy = -0.164, n = 13)，配對間的遺傳相似度顯著低於隨機配對(RANDOM PAIRS) (bootstrap, 平均Rxy = -0.046, p = 0.005, 雄性成鳥n = 36, 雌性成鳥n = 34)。被偶外交配的配對(CUCKOLDED PAIRS)(平均Rxy = 0.128, n = 3)配對間的遺傳相似度則顯著高於隨機配對(bootstrap, 平均Rxy = -0.044, p = 0.023)而偶外交配的配對(EPF PAIRS)(平均Rxy = 0.016, p = 0.225, n = 3)，配對間的遺傳相似度則與隨機配對沒有顯著差異。
青背山雀的遺傳異型交配主要是根據遺傳合適性假說而來：雌性傾向與關係較遠的雄性交配。另外，當雌青背山雀與遺傳相似度高的雄性交配時，偶外交配(EPF)可能是增加子代異型合子的合適策略。在我們的研究中，有偶外交配子代巢中原生子代的平均異型合子(平均SH值=0.905, n=3, 平均IR值=0.032, n=3)是顯著低於沒有偶外交配巢中子代的平均異型合子(bootstrap, 平均SH值=1.038, 平均IR值=-0.096, n=18, p<0.001)但有偶外交配子代巢中偶外交配子代的平均異型合子，(平均SH值=1.057, p=0.242, 平均IR值=-0.101, p=0.412, n=4)，則與沒有偶外交配巢中子代的平均異型合子沒有顯著差異。這個結果說明，雌性經由遺傳上較不相似的雄性偶外交配，來提高子代的異型合子，符合遺傳合適性假說。
此外，將青背山雀繁殖巢中有偶外交配子代的巢數比例(18%)與溫帶地區的近似種－白頰山雀(P. major)做比較，發現演化過程中的特徵保留可能是影響（亞）熱帶地區鳥類偶外交配比例的重要原因。

Genetic compatibility hypothesis suggests that females should produce offspring with the most adaptive genetic combination by mating with a most compatible male. If fitness is correlated with the heterozygosity of a set of genes or entire genome, the hypothesis predicts that females should mate with genetically dissimilar males to produce offspring with higher heterozygosity. To test the genetic compatibility hypothesis, we used nine polymorphic microsatellite loci to measure the heterozygosity and genetic similarity between mated pairs of the socially monogamous Green-backed Tit (Parus monticolus) at central Taiwan from 2002 to 2004. We found that genetic similarity between pairs that did not engage in extra-pair fertilization (EPF) (mean Rxy= -0.164, n= 13) was lower than that expected in random (bootstrap mean Rxy= -0.046, p = 0.005, n= 36 and 34 for adult males and females respectively). In contrast, genetic similarity between cuckolded pairs (mean Rxy= 0.128, n= 3) was significantly higher than that expected in random (bootstrap mean Rxy= -0.044, p = 0.023), but not for the EPF pairs (mean Rxy= 0.016, p= 0.255, n =3). Genetically disassortative mating of Green-backed Tits thus supported the major collateral of the genetic compatibility hypothesis: females tend to mate with distantly related males. In addition, EPF may serve as an adaptive strategy for female tits having genetically similar social mate, which was supported in our study that the mean heterozygosity of WPY of EPF broods (average SH and IR per brood= 0.905 and 0.0.32, respectively; n= 3) was significantly lower than that of broods without EPF (bootstrap mean SH and IR per brood= 1.038 and -0.096 respectively, n= 18; p<0.001 for both SH and IR), whereas that of EPY was not (average SH and IR per brood= 1.057 and -0.101, respectively; n= 4; p= 0.242 and 0.412 for SH and IR). It indicates female tendency to elevate heterozygosity of offspring by engaging in EPF with genetically dissimilar males, as predicted by the genetic compatibility hypothesis.
Because frequency of extra-pair fertilizations of the Green-backed Tit (18% of total broods) is compatible to that of its temperate relative, the Great Tit (P. major), we proposed that the phylogenetic constrain might play an important role to shape the frequency of extra-pair fertilization in the (sub)tropical birds .

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