Molecular cytogenetic insights into the evolution of the epiphytic genusLepismium(Cactaceae) and related genera

Abstract

Changes in chromosome structure and number play an important role in plant evolution. This was investigated in the Neotropical epiphytic cacti: all Lepismium spp. and some related Rhipsalis spp. Both genera have species with disjunct distributions between the paranas of south-eastern Brazil and north-eastern Argentina and the yungas forests of the eastern Andes. Karyotypes, fluorescent banding and fluorescence in situ hybridization (FISH) studies using rDNA probes were performed. A time-calibrated phylogenetic tree was generated to place the karyological information and biogeographical history in an explicit evolutionary context. All species were 2n = 22 and showed symmetrical karyotypes comprising only metacentric chromosomes of similar sizes. The heterochromatin bands were always associated with chromosome satellites coinciding with the location and number of the 18S-5.8S-26S rDNA loci. The 5S rDNA loci had more heterogeneous profiles with one or two loci per haploid genome. Phylogenetic analysis suggested an ancient duplication event of the 5S rDNA loci and more recent post-speciation translocation and deletion events. These genome restructurings are estimated to have occurred approximately 13.98 Mya in the middle Miocene, after Lepismium and Rhipsalis diverged. The ancestor of Lepismium may have had a similar karyotype to L. lumbricoides and the Rhipsalis spp. (i.e. one 5S locus on chromosome 2). Both genera hypothetically originated in the yungas (north-eastern Argentina and southern Bolivia), but diversification of the Lepismium crown group probably originated from populations with duplicated 5S loci in the parana forests of south-eastern Brazil (8.70 Mya in the late Miocene). Two migration events between the yungas and parana forests were suggested to explain the extant distribution of Lepismium spp. These results make Lepismium a model system for the study of the complex chromosomal evolution in plants.

Karyotype characterization and evolution of chromosome number in Cactaceae with special emphasis on subfamily Cactoideae

“…Fluorescent in situ hybridization provides additional chromosome markers for these analyses. The the physical mapping of 45S rDNA exhibited a conserved pattern, with the number of sites strictly correlated with species ploidy: two sites in the diploid species, four sites in the tetraploid species (Peñas et al 2009;Moreno et al 2015;Castro et al 2016; result presented here). All 45S rDNA sites were terminally localized and co-localized with CMA + bands, in agreement with the most common situation observed in plants (Lima- de-Faria 1980;Roa & Guerra 2012).…”

“…The presence of both terminal and proximal CG-rich fluorescent bands detected here seems to be a common characteristic among Cactoideae species (Peñas et al 2008;Moreno et al 2015). Pereskia did not have proximal CMA + bands (Peñas et al 2014;Castro et al 2016), suggesting that the amplification of CG-rich heterochromatin occurred in the common ancestor of Opuntioideae and Cactoideae.…”

“…However, a complete survey of chromosome data for Cactaceae is still missing, preventing a more comprehensive identification of the main chromosome rearrangements in an evolutionary context. Symmetrical karyotypes and 2n = 22 are conserved characteristics across the family (Stefano et al 2011;Majure et al 2012;Castro et al 2013;Moreno et al 2015). Nevertheless, the homogeneous karyotypes of cacti, when submitted to differential staining and rDNA sequence localization, reveal to be variable and typically informative.…”

“…The genera Opuntia and Brasiliopuntia also exhibit pericentromeric CMA + bands, whereas the related genus Tacinga is characterized by the absence of such pericentromeric CMA + bands (Castro et al 2016). Duplicated 5S rDNA sites are characteristic of most Lepismium clades, but this is absent in the more derived clades (Moreno et al 2015).…”

rDNA Loci Evolution in the Genus Glechoma (Lamiaceae)

“…Chromosome number changes have long been used to draw evolutionary and systematic inferences, but modern molecular cytogenetic techniques allow us to trace chromosomal structural changes in a phylogenetic context [5–8]. Molecular cytogenetic mapping of 5S and 35S rDNA loci using FISH (fluorescence in situ hybridisation) has proved useful for identifying the direction of chromosomal change in closely related species, even when chromosome numbers are conserved and chromosomes are constant in morphology [9–13]. …”