BackgroundIntraspecific variation in chromosome structure may cause genetic incompatibilities and thus provides the first step in the formation of species. In ants, chromosome number varies tremendously from 2n = 2 to 2n = 120, and several studies have revealed considerable variation in karyotype within species. However, most previous studies were limited to the description of chromosome number and morphology, and more detailed karyomorphometric analyses may reveal additional, substantial variation. Here, we studied karyotype length, genome size, and phylogeography of five populations of the fungus-farming ant Trachymyrmex holmgreni in order to detect potential barriers to gene flow.ResultsChromosome number and morphology did not vary among the five populations, but karyotype length and genome size were significantly higher in the southernmost populations than in the northern populations of this ant. Individuals or colonies with different karyotype lengths were not observed. Karyotype length variation appears to result from variation in centromere length.ConclusionT. holmgreni shows considerable variation in karyotype length and might provide a second example of centromere drive in ants, similar to what has previously been observed in Solenopsis fire ants. Whether this variation leads to genetic incompatibilities between the different populations remains to be studied.Electronic supplementary materialThe online version of this article (10.1186/s12862-018-1247-5) contains supplementary material, which is available to authorized users.

The mechanisms and processes driving change and variation in the genome size (GS) are not well known, and only a small set of ant species has been studied. Ants are an ecologically successful insect group present in most distinct ecosystems worldwide. Considering their wide distribution and ecological plasticity in different environmental contexts, we aimed to expand GS estimation within Formicidae to examine distribution patterns and variation in GS and base composition and to reconstruct the ancestral state of this character in an attempt to elucidate the generalized pattern of genomic expansions. Genome size estimates were generated for 99 ant species, including new GS estimates for 91 species of ants, and the mean GS of Formicidae was found to be 0.38 pg. The AT/GC ratio was 62.40/37.60. The phylogenetic reconstruction suggested an ancestral GS of 0.38 pg according to the Bayesian inference/Markov chain Monte Carlo method and 0.37 pg according to maximum likelihood and parsimony methods; significant differences in GS were observed between the subfamilies sampled. Our results suggest that the evolution of GS in Formicidae occurred through loss and accumulation of non-coding regions, mainly transposable elements, and occasionally by whole genome duplication. However, further studies are needed to verify whether these changes in DNA content are related to colonization processes, as suggested at the intraspecific level.

Ants (Formicidae) present considerable diversity in chromosome numbers, which vary from n = 1 to n = 60, although this variation is not proportional to that in genome size, for which estimates range from 0.18 pg to 0.77 pg. Intraspecific variation in the chromosome number and karyotype structure has been reported among species, although the variation among populations of the same species has received much less attention, and there are few data on genome size. Here, we studied the karyotype length and genome size of different populations of the fungus-farming ants Mycetophylax conformis (Mayr, 1884) and Mycetophylax morschi (Emery, 1888). We also provide remarks on procedure for the estimation of ant genome size by Flow Cytometry (FCM) analysis. Chromosome number and morphology did not vary among the populations of M. conformis or the cytotypes of M. morschi, but karyotype length and genome size were significantly distinct among the populations of these ants. Our results on the variation in karyotype length and genome size among M. morschi and M. conformis populations reveal considerable diversity that would be largely overlooked by more traditional descriptions of karyotypes, which were also supported by the estimates of genome size obtained using flow cytometry. Changes in the amount of DNA reflect variation in the fine structure of the chromosomes, which may represent the first steps of karyotype evolution and may occur previously to any changes in the chromosome number.

No abstract

No abstract