State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, 650223, Yunnan, People's Republic of China.
Key Laboratory of Forensic Sciences, Ministry of Health, Xi'an Jiaotong University, Xi'an, 710061, People's Republic of China.
BMC Ecol Evol. 2022 Mar 3;22(1):23. doi: 10.1186/s12862-022-01979-x.
Previous cytogenetic studies show that the karyotypes of species in Ciconiiformes vary considerably, from 2n = 52 to 78. Their karyotypes include different numbers of small to minute bi-armed chromosomes that have evolved probably by fusions of two ancestral microchromosomes, besides macrochromosomes and dot-like microchromosomes. However, it is impossible to define the inter-species homologies of such small-sized bi-armed chromosomes based on chromosome morphology and banding characteristics. Although painting probes from the chicken (Gallus gallus, GGA) chromosomes 1-9 and Z have been widely used to investigate avian chromosome homologies, GGA microchromosome probes are rarely used in these studies because most GGA microchromosome probes generated by flow sorting often contain multiple GGA microchromosomes. In contrast, the stone curlew (Burhinus oedicnemus, BOE, Charadriiformes) has an atypical low diploid chromosome number (42) karyotype and only 4 pairs of dot-like microchromosomes; a set of chromosome-specific painting probes that cover all BOE chromosomes has been generated. To get a genome-wide view of evolutionary chromosomal rearrangements in different lineages of Ciconiiformes, we used BOE painting probes instead of GGA painting probes to analyze the karyotypes of three ciconiiform species belonging to two different families: the eastern grey heron (Ardea cinerea, ACI, 2n = 64, Ardeidae), the little egret (Egretta garzetta, EGA, 2n = 64, Ardeidae) and the crested ibis (Nipponia nippon, NNI, 2n = 68, Threskiornithidae).
BOE painting probes display the same hybridization pattern on chromosomes of ACI and EGA, while a different hybridization pattern is observed on chromosomes of NNI. BOE autosome probes detected 21 conserved homologous segments and 5 fusions on the sixteen pairs of recognizable chromosomes of ACI and EGA, while 16 conserved homologous segments and 4 fusions were found on the twelve pairs of recognizable chromosomes of NNI. Only a portion of smaller bi-armed chromosomes in the karyotypes of the ciconiiform species could have evolved from fusions of ancestral microchromosomes. In particular BOE 5, which is the result of a fusion between two segments homologous to GGA 7 and 8 respectively, was retained also as either a single chromosome in ACI (ACI 5) and EGA (EGA 5) or had fused with a part of the BOE 10 equivalent in NNI (NNI 5).
Our painting results indicate that different chromosome rearrangements occur in different ciconiiform lineages. Some of the small-sized bi-armed chromosomes in ACI, EGA and NNI are derived from the fusions of two microchromosomes, indicating that microchromosome fusions play an important role in ciconiiform chromosome evolution. The fusion segment homologous to GGA 7 and 8 is a potential cytogenetic signature that unites Ardeidae and Threskiornithidae.
先前的细胞遗传学研究表明，鹳形目物种的核型差异很大，从 2n=52 到 78。它们的核型包括不同数量的小到微小的双着丝粒染色体，这些染色体可能是通过两个祖先微染色体的融合进化而来的，除了大染色体和点状微染色体之外。然而，基于染色体形态和带型特征，不可能定义这些小双着丝粒染色体的种间同源性。尽管来自鸡（Gallus gallus，GGA）染色体 1-9 和 Z 的涂染探针已被广泛用于研究鸟类染色体同源性，但 GGA 微染色体探针在这些研究中很少使用，因为大多数通过流式分选产生的 GGA 微染色体探针通常包含多个 GGA 微染色体。相比之下，丘鹬（Burhinus oedicnemus，BOE，Charadriiformes）具有典型的低二倍体染色体数（42）核型，仅包含 4 对点状微染色体；已经生成了一套涵盖所有 BOE 染色体的染色体特异性涂染探针。为了全面了解鹳形目不同谱系的进化染色体重排，我们使用 BOE 涂染探针代替 GGA 涂染探针分析了属于两个不同科的三种鹳形目物种的核型：东方苍鹭（Ardea cinerea，ACI，2n=64，Ardeidae）、小白鹭（Egretta garzetta，EGA，2n=64，Ardeidae）和朱鹮（Nipponia nippon，NNI，2n=68，Threskiornithidae）。
BOE 涂染探针在 ACI 和 EGA 的染色体上显示出相同的杂交模式，而在 NNI 的染色体上观察到不同的杂交模式。BOE 常染色体探针在 ACI 和 EGA 的可识别的 16 对染色体上检测到 21 个保守同源片段和 5 个融合，而在 NNI 的 12 对可识别染色体上发现 16 个保守同源片段和 4 个融合。只有一小部分小双着丝粒染色体可能是由祖先微染色体的融合进化而来的。特别是 BOE 5，它是两个分别与 GGA 7 和 8 同源的片段融合的结果，也保留为 ACI（ACI 5）和 EGA（EGA 5）中的单个染色体，或者与 NNI 中与 BOE 10 相当的一部分融合（NNI 5）。
我们的涂染结果表明，不同的鹳形目谱系发生了不同的染色体重排。ACI、EGA 和 NNI 中的一些小双着丝粒染色体是由两个微染色体融合而来的，这表明微染色体融合在鹳形目染色体进化中起着重要作用。与 GGA 7 和 8 同源的融合片段是将 Ardeidae 和 Threskiornithidae 联系在一起的潜在细胞遗传学特征。
