The Role of Fusion in Ant Chromosome Evolution: Insights from Cytogenetic Analysis Using a Molecular Phylogenetic Approach in the Genus Mycetophylax

Abstract

Among insect taxa, ants exhibit one of the most variable chromosome numbers ranging from n = 1 to n = 60. This high karyotype diversity is suggested to be correlated to ants diversification. The karyotype evolution of ants is usually understood in terms of Robertsonian rearrangements towards an increase in chromosome numbers. The ant genus Mycetophylax is a small monogynous basal Attini ant (Formicidae: Myrmicinae), endemic to sand dunes along the Brazilian coastlines. A recent taxonomic revision validates three species, Mycetophylax morschi, M. conformis and M. simplex. In this paper, we cytogenetically characterized all species that belongs to the genus and analyzed the karyotypic evolution of Mycetophylax in the context of a molecular phylogeny and ancestral character state reconstruction. M. morschi showed a polymorphic number of chromosomes, with colonies showing 2n = 26 and 2n = 30 chromosomes. M. conformis presented a diploid chromosome number of 30 chromosomes, while M. simplex showed 36 chromosomes. The probabilistic models suggest that the ancestral haploid chromosome number of Mycetophylax was 17 (Likelihood framework) or 18 (Bayesian framework). The analysis also suggested that fusions were responsible for the evolutionary reduction in chromosome numbers of M. conformis and M. morschi karyotypes whereas fission may determines the M. simplex karyotype. These results obtained show the importance of fusions in chromosome changes towards a chromosome number reduction in Formicidae and how a phylogenetic background can be used to reconstruct hypotheses about chromosomes evolution.

Cytogenetic data on the agro-predatory ant Megalomyrmex incisus Smith, 1947 and its host, Mycetophylax conformis (Mayr, 1884) (Hymenoptera, Formicidae)

“…Here we evaluated the chromosome counts of Mycetophylax conformis from Bahia, Brazil, the host of the social parasite Megalomyrmex incisus (Cardoso et al 2016). The studied population is located 1,000 km from the northern population that was cytogenetically analyzed by Cardoso et al (2014). The diploid number of chromosomes and their morphology did not differ from previously characterized populations, showing 2n=30 with the karyotype comprising metacentric and submetacentric chromosomes.…”

“…Four distinct karyotypes have been reported for Mycetophylax , from three valid species: Mycetophylax morschi (Emery, 1888) harbors two cytotypes, 2n=26 and 2n=30, whereas Mycetophylax simplex (Emery, 1888) and Mycetophylax conformis harbor 2n=36 and 2n=30, respectively (Cardoso et al 2014). Here we evaluated the chromosome counts of Mycetophylax conformis from Bahia, Brazil, the host of the social parasite Megalomyrmex incisus (Cardoso et al 2016).…”

“…Mycetophylax conformis was karyotyped to determine whether it has the same chromosome number as populations characterized by Cardoso et al (2014), as divergent chromosome numbers were already found in some congeneric species. Larvae obtained from the colonies maintained in the laboratory were used for karyotype characterization.…”

“…This high karyotype diversity seems to be correlated to ant diversification, which currently comprises nearly 14,000 described species in 21 subfamilies (Agosti and Johnson 2016). Cytogenetic data are available for about 750 ant species (Lorite and Palomeque 2010), with more data accumulating rapidly; this information has advanced our understanding of ant systematics and evolution (Mariano et al 2012, Cristiano et al 2013, Cardoso et al 2014). However, with the growing number of ant species, more cytogenetic studies are needed to reveal the extent of chromosome diversity.…”

Cytogenetic data on six leafcutter ants of the genus Acromyrmex Mayr, 1865 (Hymenoptera, Formicidae, Myrmicinae): insights into chromosome evolution and taxonomic implications

“…Nowadays, ant cytogenetics is a rapidly developing research field (Delabie et al 2012). Cytogenetic data on fungus-growing ants with information for at least the chromosome number and morphology are available at present for 38 taxa (reviewed in Barros et al 2011, Cristiano et al 2013, Barros et al 2013, 2014a, 2014b, Cardoso et al 2014, Barros et al 2015), corresponding to about 10% of described species (Brandão et al 2011). In some ant genera, e.g.…”