Cytogenetic studies on populations of Camponotus rufipes (Fabricius, 1775) and Camponotus renggeri Emery, 1894 (Formicidae: Formicinae)

Abstract

Two valid ant species, Camponotus rufipes and Camponotus renggeri, have recently been the subject of a broad discussion with reference to taxa synonymization. Both species are quite common among the Neotropical myrmecofauna and share some unique traits, such as the shape of the scape and the pilosity patterns of the tibiae and scapes. A single morphological trait can help distinguish these species; however, only a combination of different approaches can enlighten our view of the complex phylogenetic relationships prevailing in the different populations of these two taxa. Therefore, focusing on the taxonomic issues concerning these two species, a cytogenetic survey including 10 populations of C. rufipes and two populations of C. renggeri was performed. In order to better understand the extent of the relationship between C. rufipes and C. renggeri, two common Neotropical Camponotus species, C. atriceps and C. cingulatus were taken as outgroups. All four species of Camponotus that were studied had 2n = 40 chromosomes (4sm+34st+2t); however, the abundance of chromosome rearrangements observed, combined with several chromosome markers, suggest that C. rufipes and C. renggeri are two good distinct species although closely related. The already reported chromosome translocation 2n = 39 (1m+4sm+32st+2t) for C. rufipes has been found in different populations as in the unprecedented chromosome inversions found both in C. rufipes and in C. renggeri populations. Within the C. renggeri chromosome inversions, both the heterozygous state 2n = 40 (1m+3sm+34st+2t) and the homozygous state, 2n = 40 (2m+2sm+34st+2t) were identified. However, only heterozygous specimens for chromosome inversions were found among C. rufipes, with karyotype configurations distinct from those found in C. renggeri, with 2n = 40 (1m+4sm+34st+2t). None of the populations studied showed signs of mosaic individuals. With respect to rDNA clusters, the 18S rDNA seemed to be more restricted inside the genome, as C. renggeri showed four 18S rDNA clusters, whereas, C. rufipes, C. atriceps, and C. cingulatus showed only two clusters. The chromosome locations of the 5S rDNA clusters were pointed for the first time in Formicidae, and showed itself to be more widely spread over the genome. By combining different chromosome banding approaches it was possible to demonstrate the crucial importance that chromosome inversions played on the karyotype evolution within these ants. The results also showed that chromosome translocations might be a consequence of the chromatin dynamic condition observed among Camponotus species. The homozygosis condition found in a C. renggeri from a Brazilian savanna population for chromosome inversions and the contrasting heterozygous condition for a different kind of chromosome inversion in C. rufipes from the Brazilian coastal rainforest, opens the window for a chromosome race hypothesis within the group C. renggeri and C. rufipes. The wide distribution, rich ecological interactions, genetic diversity, and morphological variability among C. renggeri and C. rufipes justify questioning of the actual taxonomic status of these species. The answer of this puzzle is clear when observing the number of 18S rDNA clusters of these ants, as C. rufipes has only two clusters whereas C. renggeri has four.

Fig 1. Karyotypes of Camponotus (Myrmothrix) and its localities.

a) C. atriceps from Lavras; b) C. cingulatus from Viçosa; c) C. cingulatus from Rio de Janeiro; d) C. renggeri from Macapá; e) C. renggeri from Nova Mutum; f) C. rufipes from Lavras; g) C. rufipes from Viçosa; h) C. rufipes from Rio de Janeiro; i) C. rufipes from Petrópolis; j) C. rufipes from Ponte Nova; k) C. rufipes from Ubá; l) C. rufipes from Curitiba; m) C. rufipes from Urucânia. Bar = 5μm.

Fig 2. Chromosome rearrangements of Camponotus (Myrmothrix).

a) C. renggeri with heterozygous chromosome inversion from Nova Mutum (2n = 40); b) C. renggeri with homozygous chromosome inversion from Nova Mutum (2n = 40); c) C. rufipes with heterozygous chromosome translocation from Lavras (2n = 39); d) C. rufipes with heterozygous chromosome translocation from Ponte Nova (2n = 39); e) C. rufipes with heterozygous chromosome inversion from Ponte Nova (2n = 40); f) C. rufipes with heterozygous chromosome translocation from Ubá (2n = 39); g) C. rufipes with heterozygous chromosome translocation from Viçosa (2n = 39); h) C. rufipes with heterozygous chromosome translocation from Curitiba (2n = 39). ✳—Rearranged chromosomes. Bar = 5μm.

Fig 3. Haploid karyotypes from males of Camponotus (Myrmothrix) and its localities.

a) C. cingulatus from Rio de Janeiro; b) C. renggeri from Macapá; c) C. rufipes from Rio de Janeiro; d) C. rufipes from Ubá; e) C. rufipes from Curitiba; f) C. rufipes with heterozygous translocation from Viçosa (n = 19). The box points a chromosome result of a translocation. Bar = 5μm.

Fig 4. C-Banded metaphases of Camponotus (Myrmothrix).

a) C. atriceps from Lavras; b) C. cingulatus from Rio de Janeiro (haploid male); c) C. renggeri from Nova Mutum; d) C. rufipes from Lavras. Bar = 5μm.

Fig 5. Fluorochrome treated metaphases of Camponotus (Myrmothrix).

(a, b) Camponotus rufipes; (c, d) C. renggeri; (e, f) C. atriceps; (g, h) C. cingulatus stained with DAPI and CMA₃ respectively. Arrows point GC-rich/AT-poor regions. Bar = 5μm.

Fig 6. Fluorochrome treated metaphases of Camponotus (Myrmothrix) with chromosome rearrangements.

All images are stained respectively with DAPI and CMA₃. (a, b) C. renggeri with homozygous inversion (2n = 40); (c, d) C. rufipes with heterozygous translocation or hybridism (2n = 40); (e, f) C. renggeri with heterozygous inversion (2n = 40); (g, h) C. rufipes with heterozygous translocation (2n = 40). Arrows point GC-rich/AT-poor regions. Arrowheads point the rearranged chromosome. Bar = 5μm.

Fig 7. Camponotus (Myrmothrix) metaphases submitted to the FISH technique for detecting 18S and 5S rDNA clusters.

(a, b) C. cingulatus from Viçosa; (c, d) C. renggeri from Nova Mutum; (e, f) C. rufipes from Viçosa showing 18S rDNA and 5S rDNA respectively; g) C. renggeri from Macapá showing 18S rDNA clusters; h) C. rufipes with heterozygous chromosome translocation (2n = 39) from Viçosa showing 18S rDNA clusters. Bar = 5μm.

Fig 8. NOR banding metaphases of Camponotus (Myrmothrix).

a) C. renggeri from Nova Mutum; b) C. rufipes from Lavras. Bar = 5μm.