Niche conservatism, divergence and polyploidy inSennaseriesAphyllae(Fabaceae: Caesalpinioideae) from arid zones of South America

Abstract

This paper aims to integrate cytogenetic and ecological approaches into a phylogenetic framework to better understand the evolution and systematics of Senna series Aphyllae, an endemic group of arid and semi-arid regions of South America. We counted the chromosome numbers at the population level in three species of series Aphyllae using a conventional technique (GIEMSA) and in situ hybridization (FISH). Phylogenetic reconstructions and inference of chromosome evolution were based on four plastid DNA regions and one nuclear marker. A principal component analysis was carried out using environmental data. Finally, ecological niche models (ENM) were implemented: (1) to identify the abiotic factors that constraint the potential geographical distribution of series Aphyllae and (2) to understand the ecological drivers for species diversification. The basic chromosome number is x = 12. Senna aphylla showed three cytotypes with (2n = 24, 48 and 60) and S. pachyrrhiza has two cytotypes (2n = 24 and 48). Senna crassiramea appeared to be polyploid with 2n = 48. In diploid S. pachyrrhiza, two 45S and three 5S rDNA sites were reported. In tetraploid S. crassiramea, four 45S and four 5S rDNA sites were detected. The ENM and ecological speciation tests suggest that in series Aphyllae speciation was sometimes associated with conservatism and sometimes with niche divergence. Our results contribute to the evolutionary knowledge of Neotropical legumes and understanding of drivers of diversification in arid regions in South America.

FISH Mapping of Four Senna Species Revealing Genome Dynamics during Species Diversification of the Genus

“…The chromosomal variation, encompassing dysploidy and polyploidy with chromosome numbers of 2n = 26, 28, 48, and 56, observed in the four Senna species investigated here was consistent with previous reports (Souza and Benko-Iseppon, 2004;Rice et al, 2015;Robbiati et al, 2022). However, our study revealed an intriguing correlation FISH Mapping of Four Senna Species Revealing Genome Dynamics during Species Diversification of the Genus between the chromosomal variation in this case and rearrangements of ribosomal DNA and telomeric repeat sites in…”

“…Two 5S rDNA signals were detected in all diploids, whereas tetraploids had four signals. However, an increase in 5S rDNA signals was observed in two diploid Senna species with x = 12: S. pachyrrhiza and S. uniflora, each exhibiting three to four chromosomes bearing 5S rDNA signals, (Nguyen et al, 2021;Robbiati et al, 2022).…”

“…During sexual polyploidization, meiotic restitution or non-reduction results in unreduced or 2n gametes, respectively, and tetraploid individuals are born by the fusion between two diploid gametes (Brownfield and Köhler, 2011). Tetraploid cytotypes exhibit twice the chromosomal complement size of diploid cytotypes (Robbiati et al, 2022).…”

“…Polyploidy has been observed in S. rugosa with 2n = 42, 56, and 112 (Resende et al, 2014), whereas descending dysploidy has been exemplified by different basic chromosome numbers ranging from x = 11 to x = 14 (Souza and Benko-Iseppon, 2004). In addition, the series Aphyllae exhibited three cytotypes, 2n = 24, 48, and 60, in S. aphylla, whereas S. pachyrrhiza displayed two cytotypes, 2n = 24 and 48 (Robbiati et al, 2022). The most common diploid chromosome number in Senna species is 2n = 28, with a basic chromosome number of x = 14 (Pellerin et al, 2019).…”

“…However, all of these studies focused on diploid Senna species. Although Robbiati et al (2022) undertook the FISH mapping of the tetraploid S. crassiramea species, their findings did not clearly elucidate the variation in both the chromosome number and structure. FISH was performed using ribosomal DNA (5S and 45S rDNAs) and telomeric repeat probes to compare the chromosomal compositions and structures of four Senna species: S. artemisioides, S. pallida, S. obtusifolia, and S. aphylla.…”