Heterochromatin and microsatellites detection in karyotypes of four sea turtle species: Interspecific chromosomal differences

Abstract The wide variation in size and content of eukaryotic genomes is mainly attributed to the accumulation of repetitive DNA sequences, like microsatellites, which are tandemly repeated DNA sequences. Sea turtles share a diploid number (2n) of 56, however recent molecular cytogenetic data have shown that karyotype conservatism is not a rule in the group. In this study, the heterochromatin distribution and the chromosomal location of microsatellites (CA)n, (GA)n, (CAG)n, (GATA)n, (GAA)n, (CGC)n and (GACA)n in Chelonia mydas, Caretta caretta, Eretmochelys imbricata and Lepidochelys olivacea were comparatively investigated. The obtained data showed that just the (CA)n, (GA)n, (CAG)n and (GATA)n microsatellites were located on sea turtle chromosomes, preferentially in heterochromatic regions of the microchromosomes (mc). Variations in the location of heterochromatin and microsatellites sites, especially in some pericentromeric regions of macrochromosomes, corroborate to proposal of centromere repositioning occurrence in Cheloniidae species. Furthermore, the results obtained with the location of microsatellites corroborate with the temperature sex determination mechanism proposal and the absence of heteromorphic sex chromosomes in sea turtles. The findings are useful for understanding part of the karyotypic diversification observed in sea turtles, especially those that explain the diversification of Carettini from Chelonini species.

摘要：真核生物基因组的大小与组成存在广泛差异，这主要归因于重复DNA序列的累积，其中微卫星（microsatellite）便是一类串联重复DNA序列。海龟的二倍体染色体数（2n）均为56，但近期的分子细胞遗传学研究表明，核型保守性（karyotype conservatism）并非该类群的普遍特征。本研究对绿海龟（Chelonia mydas）、蠵龟（Caretta caretta）、玳瑁（Eretmochelys imbricata）及榄蠵龟（Lepidochelys olivacea）的异染色质（heterochromatin）分布情况，以及微卫星序列(CA)n、(GA)n、(CAG)n、(GATA)n、(GAA)n、(CGC)n与(GACA)n的染色体定位进行了比较分析。研究结果显示，仅(CA)n、(GA)n、(CAG)n及(GATA)n这四类微卫星序列可在海龟染色体上检测到，且优先定位在微染色体（microchromosome，mc）的异染色质区域。异染色质与微卫星位点的定位存在变异，尤其是在大染色体的部分着丝粒旁区域，这印证了海龟科（Cheloniidae）物种存在着丝粒重定位（centromere repositioning）现象的假说。此外，微卫星定位的研究结果也支持了海龟的温度依赖型性别决定（temperature sex determination）机制假说，以及其不存在异形性染色体（heteromorphic sex chromosomes）的结论。本研究结果有助于解析海龟类群中观察到的部分核型分化现象，尤其是阐释了蠵龟族（Carettini）与海龟族（Chelonini）物种的分化机制。