Karyotype evolution in ants (Hymenoptera: Formicidae), with a review of the known ant chromosome numbers

Ants (Hymenoptera: Formicidae) constitute a diversified insect group with more than 12,000 species described. Like other hymenopterans, they are haplodiploid whereby fertilized eggs develop into diploid females (workers and queens) whereas unfertilized eggs develop into haploid males. A large number of species have been cytogenetically studied. The chromosome number is currently known for more than 750 species. All these data are summarized in this paper. Formicidae is one of the insect groups with the most variable chromosome number. The haploid chromosome numbers are known to range from n = 1 to n = 60. This chromosome diversity suggests that karyotype modifications have accompanied ant diversification. Karyotype evolution has followed chromosome-mutation processes able to change not only chromosome number but also chromosome morphology. We review the different chromosome mutations observed in ants and the possible role of such mutations in karyotype evolution in these insects, and we examine the hypotheses proposed to explain how this karyotype evolution may have occurred. Among chromosome rearrangements, Robertsonian centric fusions and fissions, besides inversions and translocations, seem to be the main processes that generate changes in ant karyotypes. Other processes altering chromosome numbers, such as polyploidy or aneuploidy, do not appear to be important in ant evolution. Ant subfamilies present different levels of variation in relation to chromosome number. The highest variation has been found in primitive subfamilies such as Ponerinae (n = 3 - 60) and Myrmeciinae (n = 1 - 47) whereas in less primitive subfamilies the chromosome numbers are less variable, as in Dolichoderinae (n = 5 - 16), Formicinae (n = 8 28), and Myrmicinae (n = 4 - 35). Few data are available for other subfamilies. Primitive ants present not only the highest range of variation in chromosome number but also the most complex chromosome polymorphisms. In contrast, less primitive genera show lower variation in chromosome number, and generally only simple polymorphisms have been detected. We conclude with an outlook on future research avenues.