Detecting Mechanisms of Karyotype Evolution in Heterotaxis (Orchidaceae)

Abstract

The karyotype is shaped by different chromosome rearrangements during species evolution. However, determining which rearrangements are responsible for karyotype changes is a challenging task and the combination of a robust phylogeny with refined karyotype characterization, GS measurements and bioinformatic modelling is necessary. Here, this approach was applied in Heterotaxis to determine what chromosome rearrangements were responsible for the dysploidy variation. We used two datasets (nrDNA and cpDNA, both under MP and BI) to infer the phylogenetic relationships among Heterotaxis species and the closely related genera Nitidobulbon and Ornithidium. Such phylogenies were used as framework to infer how karyotype evolution occurred using statistical methods. The nrDNA recovered Ornithidium, Nitidobulbon and Heterotaxis as monophyletic under both MP and BI; while cpDNA could not completely separate the three genera under both methods. Based on the GS, we recovered two groups within Heterotaxis: (1) "small GS", corresponding to the Sessilis grade, composed of plants with smaller genomes and smaller morphological structure, and (2) "large GS", corresponding to the Discolor clade, composed of plants with large genomes and robust morphological structures. The robust karyotype modeling, using both nrDNA phylogenies, allowed us to infer that the ancestral Heterotaxis karyotype presented 2n = 40, probably with a proximal 45S rDNA on a metacentric chromosome pair. The chromosome number variation was caused by ascending dysploidy (chromosome fission involving the proximal 45S rDNA site resulting in two acrocentric chromosome pairs holding a terminal 45S rDNA), with subsequent descending dysploidy (fusion) in two species, H. maleolens and H. sessilis. However, besides dysploidy, our analysis detected another important chromosome rearrangement in the Orchidaceae: chromosome inversion, that promoted 5S rDNA site duplication and relocation.

Karyotype evolution in Phalaris (Poaceae): The role of reductional dysploidy, polyploidy and chromosome alteration in a wide-spread and diverse genus

“…This has been confirmed in several groups of related species [ 67 ]. Although the correlation between genome size and phenotypic dimension is often reduced when using higher phenotypic scales [ 68 ], this hypothesis is supported by the species of Phalaris . Within the genus, a trend in sterile lemma reduction was suggested [ 8 ], with members of the early diverging x = 6 lineage displaying relatively large and lanceolate sterile lemmas, followed by gradual reduction in size, culminating in almost obsolete sterile lemmas in one of the terminal x = 7 clades.…”

Karyotype diversity and genome size variation in Neotropical Maxillariinae orchids

“…F.Barros, 2n=40 and 2C=8.71 pg, was taken from Moraes et al . (); for Trigonidium egertonianum Bateman in Lindl., 2n=40 and 2C=3.44 pg, was taken from Leitch et al . (); the GS for Peristeria elata Hook., 2C=9.34, was taken from Jones et al .…”

Intrachromosomal karyotype asymmetry in Orchidaceae

“…The occurrence of asymmetric karyotypes is probably a consequence of chromosomal structural changes, especially centric fusion/fission, a very common rearrangement in Orchidaceae ( Pinheiro et al , 2009 ; Yamagishi-Costa and Forni-Martins, 2009 ; Assis et al , 2013 ; Moraes et al , 2012 , 2013 ; 2016 ). Christensonella Szlach.…”

“…This is illustrated by the double DAPI + band holding the centromere in C. fernandiana , as the remainder species present just one DAPI + band ( Koehler et al , 2008 ; Moraes et al , 2012 ). The centric fusion/fission is suggested to be the cause of the frequent dysploidy in other genera in subtribe Maxillariinae ( Moraes et al , 2012 ; 2016 ) and Orchidaceae ( Pinheiro et al , 2009 ; Felix and Guerra, 2010 ; Assis et al , 2013 ). The same can be detected in other plant groups as the genus Tristagma Poepp.…”