Chromosomal dynamics in space and time: evolutionary history of Mycetophylax ants across past climatic changes in the Brazilian Atlantic coast

Abstract

Fungus-farming ants of the genus Mycetophylax exhibit intra and interspecific chromosome variability, which makes them suitable for testing hypotheses about possible chromosomal rearrangements that endure lineage diversification. We combined cytogenetic and molecular data from Mycetophylax populations from coastal environments to trace the evolutionary history of the clade in light of chromosomal changes under a historical and geographic context. Our cytogenetic analyses revealed chromosomal differences within and among species. M. morschi exhibited three distinct karyotypes and considerable variability in the localization of 45S rDNA clusters. The molecular phylogeny was congruent with our cytogenetic findings. Biogeographical and divergence time dating analyses estimated that the most recent common ancestor of Mycetophylax would have originated at about 30 Ma in an area including the Amazon and Southern Grasslands, and several dispersion and vicariance events may have occurred before the colonization of the Brazilian Atlantic coast. Diversification of the psammophilous Mycetophylax first took place in the Middle Miocene (ca. 18-10 Ma) in the South Atlantic coast, while "M. morschi" lineages diversified during the Pliocene-Pleistocene transition (ca. 3-2 Ma) through founder-event dispersal for the Northern coastal regions. Psammophilous Mycetophylax diversification fits into the major global climatic events that have had a direct impact on the changes in sea level as well as deep ecological impact throughout South America. We assume therefore that putative chromosomal rearrangements correlated with increased ecological stress during the past climatic transitions could have intensified and/or accompanied the divergence of the psammophilous Mycetophylax. We further reiterate that "M. morschi" comprises a complex of at least three well-defined lineages, and we emphasize the role of this integrative approach for the identification and delimitation of evolutionary lineages.

Figure 1

Map of geographic distribution and sampling localities of Mycetophylax populations along Brazilian Atlantic coast. The legends of the colors and symbols are embedded in the figure. The scale bar is represented in kilometers. Adapted from Cardoso et al..

Figure 2

Pruned phylogenetic tree from Mycetophylax fungus-farming ants based on a Bayesian analysis of five nuclear protein-coding genes and their DAPI-stained karyotypes showing the FISH mapping of the 18S rDNA probe (in red). M. simplex shown in yellow branches and karyotype 2n = 36. M. conformis shown in purple branches and karyotype 2n = 30. M. morschi (lineage A) shown in red branches and karyotype 2n = 26. M. morschi (lineage B) shown in green branches and karyotype 2n = 28. M. morschi (lineage C) shown in blue branches and karyotype 2n = 30. In the karyotypes images: (M) metacentric, (SM) submetacentric, and (A) acrocentric chromosomes.

Figure 3

Phylogenetic tree based on Fossilized Birth-Death process representing the divergence time estimates along with ancestral range estimates based on three alternative biogeographic model. The arrows on the map represent a possible route of the Mycetophylax ants along the colonization of the Brazilian Atlantic coast. Pie charts showing (a) “BioGeoBEARS” analyses (model selected: BAYAREALIKE + j) on the top, (b) the BayArea model on the middle, and (c) the Bayesian Binary MCMC model on the bottom. The pie charts at the nodes represent 95% confidence intervals of the relative frequencies of the ancestral range optimizations across the restricted tree of the genus Mycetophylax. The horizontal blue bars at the nodes represent the 95% highest posterior density (HPD) intervals of the estimated node ages. The numbers on the main nodes represent the mean ages of lineage divergence as well as the confidence intervals. The scale axis bar represents million years ago (Ma). Q.: Quaternary, Plio.: Pliocene, and Plei.: Pleistocene. The orange curve represents temperature fluctuations during the past ∼35 Ma, as depicted by Zachos et al..

Figure 4

Chromosome number evolution and inferred ancestral chromosome state in the phylogenetic tree of fungus-farming ants from ChromEvol results based on Bayesian inference, chosen because it provides posterior probabilities as a statistical parameter. The numbers at the tips are the known haploid chromosome numbers of species, while “X” represents unknown number. The various colors on the branches of the tree represent the base haploid chromosome number for each node, given in the legend of the figure.