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.

Intraspecific variation in the karyotype length and genome size of fungus-farming ants (genus Mycetophylax), with remarks on procedures for the estimation of genome size in the Formicidae by flow cytometry

“…morschi cytotypes ranged from 2n = 26 in the southern populations (RS and SC), to 2n = 28 in the Bahia population (BA), and 2n = 30 in the Rio de Janeiro (RJ) population ( Fig 3 ). These findings are consistent with those of previous studies (see [ 11 , 17 , 47 ]). No numerical or morphological variation was found among the M .…”

“…The diploid numbers of all M. conformis individuals analyzed here were 2n = 30, whereas the M. morschi cytotypes ranged from 2n = 26 in the southern populations (RS and SC), to 2n = 28 in the Bahia population (BA), and 2n = 30 in the Rio de Janeiro (RJ) population (Fig 3). These findings are consistent with those of previous studies (see [11,17,47]). No numerical or morphological variation was found among the M. conformis populations, whereas the number of chromosomes and karyotype did vary among the M. morschi population, as expected.…”

“…The findings of the present study are consistent with the hypothesis that genome size varies between more ancient lineages and populations founded more recently [53]. Micolino et al [17] inferred the ancestral area of occurrence and the putative biogeographical dispersal routes of Mycethopylax from the southern grasslands of Brazil to the Atlantic coast. In this context, the Mycetophylax conformis populations from Bahia (northeastern Brazil) had smaller mean genome sizes than the populations from Rio de Janeiro, southeastern Brazil, apparently established more recently.…”

“…In this context, the Mycetophylax conformis populations from Bahia (northeastern Brazil) had smaller mean genome sizes than the populations from Rio de Janeiro, southeastern Brazil, apparently established more recently. By contrast, the genome size estimates for Mycetophylax morschi from Rio de Janeiro and Bahia were larger than those from the southern Brazilian populations (Santa Catarina and Rio Grande do Sul), and formed a sister clade with these southern populations (see Micolino et al [17]), which had smaller genomes and karyotype (2n = 26). Similar variation in chromosome length and number among populations has been observed in two social wasps of the genus Synoeca within similar area of occurrence on Brazilian Atlantic coast [55].…”

“…So far, only a few studies have investigated the chromosomal variation at the population level (e.g. [11,[16][17][18]), and even fewer have applied a karyomorphometrical approach [18][19][20]. Karyomorphometry is a technique that has been applied amply in cytogenetic studies of plants, and is often combined with estimates of genome size, i.e.…”