Challenging Our View on Meiosis - Luzula elegans, a Plant with a Holocentric Chromosome Structure, Shows Alternative Meiotic Chromatid Segregation

Species with holocentric chromosomes occur in various independent eukaryotic lineages. Holocentric chromosomes lack a primary constriction and are characterized by holokinetic kinetochores along almost the entire poleward chromatid surfaces, except at their ends. Due to this alternative chromosome structure, species with holocentric chromosomes cannot use the two-step loss of cohesion during meiosis typical for monocentric chromosomes that requires distinction between chromosome arms and sister centromeres.

Analysis of meiosis in the wood-rush Luzula elegans, a plant possessing holocentric chromosomes during mitosis, revealed a holocentric chromosome architecture and behavior also throughout meiosis. Early prophase I events are cytologically similar to those of species with monocentric chromosomes; i.e. telomere bouquet, axis and synaptonemal complex formation occur. However, in contrast to a monopolar sister centromere orientation, holokinetic sister centromeres are unfused during meiosis I. They behave as two distinct functional units, resulting in sister chromatid separation already during meiosis I. To ensure faithful haploidization homologous non-sister chromatids remain terminally linked after metaphase I, by satellite DNA enriched chromatin threads, until metaphase II. They then separate at anaphase II. Thus, an inverted sequence of meiotic sister chromatid segregation occurs.

This alternative way of meiotic progression is most likely one possible adaptation to handle a holocentric chromosome architecture and behavior during meiosis.