Ribeiro Tiago, Buddenhagen Christopher E, Thomas W Wayt, Souza Gustavo, Pedrosa-Harand Andrea
Departamento de Botânica, Centro de Biociências, Laboratório de Citogenética e Evolução Vegetal, Universidade Federal de Pernambuco, Av. Prof. Moraes Rego, s/n, Cidade Universitária, Recife, PE, 50670-901, Brazil.
Department of Biological Science, Florida State University, Tallahassee, FL, USA.
Protoplasma. 2018 Jan;255(1):263-272. doi: 10.1007/s00709-017-1154-4. Epub 2017 Aug 26.
Karyotype evolution in species with non-localised centromeres (holocentric chromosomes) is usually very dynamic and associated with recurrent fission and fusion (also termed agmatoploidy/symploidy) events. In Rhynchospora (Cyperaceae), one of the most species-rich sedge genera, all analysed species have holocentric chromosomes and their numbers range from 2n = 4 to 2n = 84. Agmatoploidy/symploidy and polyploidy were suggested as the main processes in the reshuffling of Rhynchospora karyotypes, although testing different scenarios of chromosome number evolution in a phylogenetic framework has not been attempted until now. Here, we used maximum likelihood and model-based analyses, in combination with genome size estimation and ribosomal DNA distribution, to understand chromosome evolution in Rhynchospora. Overall, chromosome number variation showed a significant phylogenetic signal and the majority of the lineages maintained a karyotype of 2n = 10 (~48% of the species), the most likely candidate for the ancestral number of the genus. Higher and lower chromosome numbers were restricted to specific clades, whilst polyploidy and/or fusion/fission events were present in specific branches. Variation in genome size and ribosomal DNA site number showed no correlation with ploidy level or chromosome number. Although different mechanisms of karyotype evolution (polyploidy, fusion and fission) seem to be acting in distinct lineages, the degree of chromosome variation and the main mechanisms involved are comparable to those found in some monocentric genera and lower than expected for a holocentric genus.
具有非定位着丝粒（全着丝粒染色体）的物种的核型进化通常非常活跃，且与反复的裂变和融合（也称为染色体组加倍/合子倍性）事件相关。在莎草科中物种最丰富的属之一刺子莞属（Rhynchospora）中，所有已分析的物种都具有全着丝粒染色体，其数目范围从2n = 4到2n = 84。染色体组加倍/合子倍性和多倍体被认为是刺子莞属核型重排的主要过程，尽管到目前为止尚未尝试在系统发育框架内测试染色体数目进化的不同情况。在这里，我们使用最大似然法和基于模型的分析，结合基因组大小估计和核糖体DNA分布，来了解刺子莞属的染色体进化。总体而言，染色体数目变异显示出显著的系统发育信号，并且大多数谱系维持2n = 10的核型（约占物种的48%），这是该属最有可能的祖先数目候选值。较高和较低的染色体数目局限于特定的分支，而多倍体和/或融合/裂变事件则出现在特定的分支中。基因组大小和核糖体DNA位点数量的变异与倍性水平或染色体数目均无相关性。尽管核型进化的不同机制（多倍体、融合和裂变）似乎在不同的谱系中起作用，但染色体变异的程度和所涉及的主要机制与一些单着丝粒属中发现的情况相当，且低于全着丝粒属的预期。
