About 53 species of Brachymeria are described for the Neotropical region ( Tavares and Araujo 2007), but cytogenetic data are available for only three species so far. Invariably, a modal diploid value of 2n = 10 has been reported, as observed in B. lasus and B. ovata (Hung 1986) and in the presently studied species. In general, the chromosomal information in parasitoid wasps of the superfamily Chalcidoidea, with few exceptions, indicates they might be divided into two groups: (1) families characterized by low chromosomal numbers (n = 5 to 6) and (2) families with higher chromosomal numbers, ranging from n = 10 to 11 (Gokhman 2022). The putative ancestor haploid number of parasitoid hymenopterans ranges from 14 a 17, indicating multiple reduction events of chromosomal numbers within some lineages, such as Eurytomidae and Encyrtidae. On the other hand the basal haploid values in Chalcidoidea varied from n = 3 to n = 11, with a predominance of n = 6 e n = 11. In this case, centric fusions are regarded as the main arrangements involved in the karyoevolution of this group (Gokhman 2013; Gokhman 2022).
In fact, most families in Chalcidoidea are characterized by relatively low chromosomal numbers, as observed in representatives of Brachymeria (Hung 1986; Gokhman 2020) and also in species of the families Aphelinidae and Perilampidae (Baldanza et al. 1999; Gokhman 2005). Furthermore, in spite of the apparent conservativeness of the diploid number in Brachymeria, variation in the karyotype formulae have already been reported in this genus (Hung 1986). In general, congeneric species share karyotypes with a predominance of metacentric pairs, but submetacentric and acrocentric chromosomes have also been reported in the three species cytogenetically analyzed (B. intermedia, B. ovata, and B. lasus), except for B. vesparum from the present study (Table 1).
Table 1
Cytogenetic data available in species of the family Chalcididae with their respective haploid (n) and diploid (2n) numbers, karyotype formulae and origin of samples.
| Species | n | 2n | Karyotypic formula | Country | References |
| Brachymeria intermedia (Ness, 1966) | 3 | 6 | 2K = 2M + 4SM | GER | Hung, 1986 |
| Brachymeria lasus (Walker, 1841) | 5 | 10 | 2K = 6M + 2SM + 2A | JPN | Hung, 1986 |
| Brachymeria ovata (Say, 1824) | 5 | 10 | 2K = 2M + 4SM + 4A | USA | Hung, 1986 |
| Brachymeria (pseudobrachymeria) vesparum (Boucek, 1992) | - | 10 | 2K = 10M | BRA | Present study |
| Dirhinus himalayanus (Westwood, 1836) | 5 | 10 | - | PH | Amalin et al.,1988 |
| Psilochalcis brevialata (Grissell & Johnson, 2001) | 6 | 12 | 2K = 8M + 4SM | USA | Johnson et al., 2001 |
The unique karyotype formula herein reported for B. vesparum when compared to the other congeneric species suggest that structural rearrangements, such as pericentric inversions, as well as centric fusions played a key role in the karyotype evolution of these insects (Fig. 3). It should be pointed out that such differences might be influenced by technical artifacts related to distinct criteria of determining chromosomal types by authors or by the differential chromatin condensation. Nonetheless, even when chromosome pairs were grouped into two main classes, i.e. meta/submetacentric and acrocentric chromosomes, to minimize putative biased results, the Brachymeria species are still differentiated by their karyotype structure inasmuch as B. vesparum, B. lasus, B. ovata, and B. intermedia present FN = 20, FN = 18, FN = 16 and FN = 12, respectively.
The genome architecture, including chromosomal structure, is directly related to the transmission of genetic traits within and among populations, thus being capable of promoting species diversification (Feder et al. 2011). As a matter of fact, differences related to chromosomal rearrangements have been directly related to speciation events (Potter et al. 2017). For instance, heterozygous inversions or translocations might affect the gametogenesis, leading to infertility or even affecting the survival of hybrid forms (Livingstone and Rieseberg 2004).
In some cases, chromosomal rearrangements are associated with recombination suppression, favoring local adaptation and the accumulation of genetic incompatibilities between evolutionary units (Ortiz-Barrientos et al. 2016), as already reported in grasshoppers (Shaw et al. 1976). Therefore, the unique chromosomal features observed in Brachymeria species suggest that the fixation of distinct karyotypic forms represent potential barriers to reproduction, acting a drivers of speciation. From a cytotaxonomic point of view, the chromosomal traits reported within Brachymeria are useful as species-specific markers, being information to the diagnosis of congeneric species.
On the other hand, in spite of the major role of heterochromatin in the karyotype diversification of several animal groups (Bitencourt et al. 2011; Tavares et al. 2021), the analysis of heterochromatin distribution in the present study was poorly informative. As a matter of fact, C-bands at pericentromeric regions are commonly reported in many species of the superfamily Chalcidoidea (Gokhman and Westendorff 2000; Gokhman 2022), while the presence of heterochromatic chromosomal arms have also been observed in some parasitoid wasps (Baldanza et al. 1999; Gokhman and Westendorff, 2000). Most likely, such conspicuous C-band represents remnants of heterochromatin from pseudoacrocentric chromosomes after fusion events, thus corroborating the role of chromosomal rearrangements in the evolution of this group of insects.
As for the composition of heterochromatin as revealed by fluorochrome staining, the species from the present study revealed GC-rich regions in two chromosome pairs, thus diverging from the common pattern (AT-rich segments) reported for most parasitoids (Baldanza et al, 1999). Nonetheless, GC+ signals havel also been described at interstitial region in the parasitoid wasps Palmistichus elaeisis and Baryscapus silvestrii (Gokhman, 2017; 2019), as well as terminal regions of all chromosomes in the karyotype of Trichospilus diatrae (Gokhman, 2017).
In addition, the mapping of nucleolus organizer regions (NORs) in karyotypic studies have also been carried out in cytogenetic studies of parasitoids (Baldanza 1999; Van Vugt et al. 2005), usually revealing single or double NORs in Chalcidoidea (Gokhman 2022). These regions can be identified by several methods, including base-specific fluorochrome staining since NORs are usually interspersed with GC-rich sites (CMA3+ signals) (Schweizer 1994). However, it should be pointed out that additional fluorochrome signals, unrelated to NORs, might be present throughout the chromosomal DNA (Gokhman 2022). Accordingly, caution is recommended before considering that the interstitial CMA3+ signals on pairs 1 and 4 of B. vesparum actually refer to NORs. In this sense, other techniques should be carried out to confirm the location of ribosomal cistrons, such as silver nitrate staining and fluorescent in situ hybridization (FISH) with rDNA probes.
Even though refined methods of chromosomal analyses have been performed (Van Vugt et al. 2005, 2009; Bolsheva et al. 2012; Gokhman et al. 2017), these reports are still scarce in parasitoid wasps (Gokhman 2010; Gebiola et al. 2012) and several taxonomic uncertainties remain to be resolved in these insects. Actually, the karyotypic data herein described for Brachymeria represents an advance in the understanding of evolutionary mechanisms and cytogenetic patterns in this genus. Moreover, chromosomal markers such as the karyotype structure and the distribution of GC-rich heterochromatic sites proved to be useful for cytotaxonomy by providing diagnostic characters. Therefore, we reinforce that similar studies should be expanded to other members of the family Chalcididae thus allowing refined evolutionary and systematic inferences in these insects, including the potential identification of cryptic forms or species complexes.