Plastid DNA Diversity Is Higher in the Island Endemic Guadalupe Cypress than in the Continental Tecate Cypress

BackgroundCallitropsis guadalupensis (Guadalupe cypress) is endemic to Guadalupe Island, Mexico, where it is the dominant species of the only forest. The species has suffered declining numbers following the introduction of goats to the island over 150 years ago. Callitropsis guadalupensis is closely related to Callitropsis forbesii (Tecate cypress), distributed in small isolated populations in mainland Baja California and southern California. The objective of the present study was to compare the genetic diversity of the island endemic to the continental species. Methodology/Principal FindingsWe measured genetic diversity in Callitropsis guadalupensis (n = 54) from Guadalupe Island and in Callitropsis forbesii (n = 100) from five populations in mainland Baja California. The plastid DNA trnS-trnG spacer and the trnL-trnF region were chosen for characterization. Thirty-four haplotypes were observed, of which six were shared between both species. One of these haplotypes was also shared with three other species, Callitropsis lusitanica, Callitropsis montana, and Callitropsis stephensonii. Haplotype diversity (h) and nucleotide diversity (π) were significantly higher for Callitropsis guadalupensis (h = 0.698, π = 0.00071) than for Callitropsis forbesii (h = 0.337, π = 0.00024). Conclusions/SignificanceCallitropsis guadalupensis shows no evidence of a founder effect or of a genetic bottleneck, and can be added to a growing list of insular species with higher genetic diversity than their mainland relatives.背景 瓜达卢佩柏（Callitropsis guadalupensis，Guadalupe cypress）为墨西哥瓜达卢佩岛特有物种，是该岛唯一森林的优势种。150余年前山羊被引入该岛后，该物种种群数量持续衰退。瓜达卢佩柏与特卡特柏（Callitropsis forbesii，Tecate cypress）亲缘关系密切，后者在北美大陆下加利福尼亚州及加利福尼亚州南部以小型孤立种群形式分布。本研究旨在比较该海岛特有种与其大陆近缘种的遗传多样性。 方法与主要结果 本研究对采集自瓜达卢佩岛的54份瓜达卢佩柏样本，以及采集自下加利福尼亚州大陆5个种群的100份特卡特柏样本开展遗传多样性检测。选取质体DNA（plastid DNA）的trnS-trnG间隔区与trnL-trnF区域进行序列分析。共鉴定得到34个单倍型（haplotype），其中6个为两个物种所共有；其中1个单倍型还同时存在于另外3个物种——葡萄牙柏（Callitropsis lusitanica）、山地柏（Callitropsis montana）与斯蒂芬森柏（Callitropsis stephensonii）中。瓜达卢佩柏的单倍型多样性（h=0.698）与核苷酸多样性（π=0.00071）均显著高于特卡特柏（h=0.337，π=0.00024）。 结论与意义 瓜达卢佩柏未表现出奠基者效应（founder effect）或遗传瓶颈（genetic bottleneck）的相关特征，该物种可被纳入遗传多样性高于大陆近缘种的海岛物种名录，且此类物种的研究案例正不断扩充。