Agmatoploidy and symploidy: a critical review

Abstract

Agmatoploidy is a type of chromosome rearrangement that involves the fragmentation of an entire chromosome complement, generating a diploid with double its original chromosome number. Agmatoploidy and other related karyotype changes, such as symploidy (the opposite change, promoted by chromosome fusion), partial agmatoploidy, polyagmatoploidy, etc., are restricted to species with holokinetic chromosomes and are assumed to play an important role in their karyotype evolution. However, a critical review of the literature shows that examples of chromosome number doubling by agmatoploidy are rare and not clearly demonstrated, while partial agmatoploidy and partial symploidy seem to be the same as ascending and descending disploidy, respectively. It is therefore proposed here that these terms should be avoided or even abolished.

Diversification and distribution patterns of Luzula sect. Luzula (Juncaceae) in the Eastern Alps: a cytogenetic approach combined with extensive herbarium revisions

“…Holocentric chromosomes alongside agmatoploidy and/ or symploidy have been observed in at least nine animal and at least five angiosperm lineages (Melters et al 2012;Bureš et al 2013), namely Cyperaceae/Juncaceae/Thurniaceae (Nordenskiöld 1951;Löve et al 1957;Hipp et al 2009;Bozek et al 2012), Chionographis (Melanthiaceae; Tanaka and Tanaka 1977), Cuscuta (Convolvulaceae; Pazy and Plitmann 1995), Droseraceae (Melters et al 2012;Kolodin et al 2018) and Myristica (Myristicaceae; Flach 1966), but might be even more common . In Luzula true agmatoploidy and symploidy were observed, whereas in other taxa only partial agmato-and/or symploidy were evidenced (Guerra 2016).…”

“…Whereas in the case of true polyploidy the monoploid GS can decrease significantly after polyploidisation, a phenomenon termed genome downsizing (Verma and Rees 1974;Leitch and Bennett 2004;Renny-Byfield et al 2013), little is known about the changes in GS in the case of agmatoploidy (Guerra 2016). Agmatoploids should have DNA contents similar to their original, non-fragmented complements (Guerra 2016), which was confirmed by early DNA content estimations based on arbitrary values obtained from photometric studies in Luzula species (Halkka 1964). However, later studies revealed quantitative changes in the DNA content of members of agmatoploid series (e.g., Barlow and Nevin 1976).…”

“…Diversification of Luzula has been strongly influenced by polyploidisation, which was an important driving force in the evolution of Angiosperms (Soltis et al 2009;Husband et al 2013;Weiss-Schneeweiss et al 2013) and is also one of the major factors affecting high mountain biodiversity in the Alps (Burnier et al 2009;Guggisberg et al 2009;Casazza et al 2012;Pachschwöll et al 2015). Contrary to true polyploidy, where GS at least initially increases linearly with the increased ploidy level, in the case of agmatoploidy (Malheiros and Gardé 1950;Löve et al 1957;Luceña and Guerra 1996) the increase in chromosome number is not correlated with an increase of GS; instead, the nuclear DNA amount is retained (Mello-Sampayo 1961;Bačič et al 2007a;Guerra 2016). Agmatoploidy is based on fission of holocentric (holokinetic) chromosomes; in contrast to the "normal type" of monocentric chromosomes, where spindle microtubules attach to a single kinetochore during mitosis or meiosis, in holocentric chromosomes they attach along the whole length through diffuse kinetochores (Bureš et al 2013).…”

“…Agmatoploidy is based on fission of holocentric (holokinetic) chromosomes; in contrast to the "normal type" of monocentric chromosomes, where spindle microtubules attach to a single kinetochore during mitosis or meiosis, in holocentric chromosomes they attach along the whole length through diffuse kinetochores (Bureš et al 2013). In addition, holocentric sister chromatids are interconnected along their whole length before anaphase disjunction and their fragmentation is thus not necessarily deleterious as the fragmented parts are successfully segregated in daughter nuclei (Nordenskiöld 1951;Haizel et al 2005;Nagaki et al 2005;Bozek et al 2012;Bureš et al 2013;Guerra 2016). The term agmatoploidy was in the past applied also for the opposite process, that is the concerted fusion of holocentric chromosomes, generating a karyotype with only half of the original chromosome number, but Luceña and Guerra (1996) introduced the term symploidy for this phenomenon.…”

“…The term agmatoploidy was in the past applied also for the opposite process, that is the concerted fusion of holocentric chromosomes, generating a karyotype with only half of the original chromosome number, but Luceña and Guerra (1996) introduced the term symploidy for this phenomenon. In addition to complete agmatoploidy and symploidy also partial agmatoploidy and symploidy have been observed, with only a subset of chromosomes in the nucleus fragmented or fussed resulting in a continuous series of chromosome numbers (Kirschner 1992a;Guerra 2016;Bureš et al 2013).…”