The species studied exhibited structural differences in their karyotypes. The chromosomes were mostly metacentric and, at a smaller proportion, submetacentric. The number 2n = 34 in females and n = 17 in males was consistent in the three species, which demonstrates a very conserved characteristic in the regular karyotype of bees from the tribe Meliponini (Tavares et al. 2017), including several species of the genus Partamona: Partamona pearsoni, (Tarelho 1973), P. ailyae, Partamona vicina, Partamona mulata, Partamona sp. (Brito-Ribon et al. 1999), Partamona seridoensis (Brito et al. 2005), Partamona nhambiquara, Partamona chapadicola, and P. rustica (Miranda 2012).
Although supernumerary chromosomes have been described in some phylogenetically related species of Partamona, such as P. helleri (Costa et al. 1992; Martins et al. 2014) and Partamona cupira (Marthe et al. 2010), the colonies sampled in the present study showed no evidence of this phenomenon.
Consistency in the chromosome morphology was found in Partamona ailyae when we compared the data obtained herein with the results described by Brito-Ribon et al. (1999). The distribution of heterochromatin, which was found in nearly all chromosomes, also had a similar pattern, with a greater quantity concentrated in the short arms of most chromosomes. The only exception was the first pair, in which most of the heterochromatin was concentrated throughout the length of the long arms and, to a lesser extent, in the pericentromeric region of the short arms. Chromosomes with euchromatic regions were found in pairs 8, 9, and 13, whereas Brito-Ribon et al. (1999) found such regions in pairs 7, 10, and 16. Despite the variation in the number of chromosomes with this euchromatic pattern, all chromosomes were morphologically submetacentric.
In P. ailyae, both pairs 3 and 14 exhibited heteromorphism regarding the presence/absence of CMA3+ markings. However, observing the karyotype of P. aff. helleri, heteromorphism was found regarding the presence/absence of CMA3+ markings in pairs 1, 5, 11, and 15 (Figs. 3e and 4c), which is a significant amount of heteromorphic markings in the chromosomes of a species of bee. Heteromorphic markings are common in other species of Meliponini, such as Trigona branneri (Costa et al. 2004) and the genus Scaptotrigona (Duarte et al. 2009). According to Fry and Salser (1977) cited by Sumner (2003), heteromorphisms in the composition of heterochromatin are related to differential amplifications of distinct segments that may be amplified in some sequences, forming heterochromatic blocks in some species, whereas different sequences are amplified in other species. In P. ailyae, two rDNA clusters were detected and were located in pair 9, in which the region corresponding to 18S was heterochromatic with conventional staining (Fig. 2a and b).
The cytogenetic characterization of P. vicina, P. mulata, and P. ailyae performed by Brito-Ribon et al. (1999) revealed very similar results to the karyotypes of the species investigated in the present study regarding the morphology and chromosome size of the first pair. It is possible that the chromosomal homeology of this pair occurs in related species in the genus Partamona.
Andrade et al. (2017) found that specimens collected in the municipalities of Presidente Tancredo Neves, Uruçuca, and Una in the state of Bahia when compared with P. ailyae through mtDNA 12S analyses were possibly of the same species, as they shared an insert of five bases in this mitochondrial region in a unique way within Partamona. This hypothesis is consistent with Camargo and Pedro (2003), who present a morphological phylogeny in which these two species are closely related and placed as sister groups. However, the cytogenetic data presented herein reveal that our samples of Partamona sooretamae and Partamona ailyae exhibited differences in their karyotypes, reinforcing the idea that these may be isolated species. In case this hypothesis is correct, the sharing of the mitochondrial sequence insertion described previously would be a synapomorphy and, alternatively, be an ancestral character. A broader investigation, however, would be necessary to verify possible occurrences in other closely related species.
The karyotype of P. aff. helleri had the most differences compared with the karyotypes of P. ailyae and P. sooretamae. It had no similarity in size or morphology in the first pair of chromosomes and this pair also exhibited CMA3 band heteromorphism that was not found in the other species. The number of CMA3+ markings was also high in P. aff. helleri, reaching 18, which is a large number in hymenopterans. However, the chromosomes of the second pair had morphological similarities to those found in P. ailyae and P. sooretamae with regard to both morphology and CMA3+ markings.
The present results suggest that P. ailyae and P. sooretamae are cytogenetically distinct. Although chromosome number was conserved, the karyotypes showed several structural differences due to variations in the content and distribution of constitutive heterochromatin as well as the number and location of 18S rDNA regions. These differences may have important evolutionary implications and contribute to interspecific differentiation. The cytogenetic characters described, for instance, are useful to the cytotaxonomic distinction of these species.