Molecular evidence for the occurrence of a new sibling species within the Anopheles (Kerteszia) cruzii complex in south-east Brazil

Anopheles cruzii has long been known as a vector of human and simian malaria parasites in southern and south-eastern Brazil [1, 2]. This species, which belongs to the subgenus Kerteszia, is found from the coast of Rio Grande do Sul State in southern Brazil to Sergipe State in north-eastern Brazil [3, 4], all along the Brazilian Atlantic forest. This forest provides an excellent environment for An. cruzii, since it is an ecosystem abundant in bromeliads, the larval habitat for this anopheline [2, 5, 6].

The possibility that An. cruzii could represent more than a single species was first supported by morphological differences observed among populations from the states of Santa Catarina and Rio de Janeiro [3]. Later it was revealed that An. cruzii is polymorphic for chromosome rearrangements [7, 8]. Differences in inversion frequencies and X chromosome banding patterns from populations in south-eastern and southern Brazil have suggested a process of incipient speciation [9, 10]. Malafronte et al[11] compared sequences of ITS2 (Internal Spacer Region 2) from several An. cruzii populations from south and south-east Brazil and found differences between sequences from different localities, although they considered premature to conclude based on their results that there are distinct sibling species in the areas investigated. Similar results were observed by Calado et al[12], using PCR-RAPD and PCR-RFLP of the ITS2 region.

Finally, isoenzyme analysis indicated two genetically isolated groups, one from Bahia State (north-eastern Brazil), and the other from south-eastern and southern Brazil (Rio de Janeiro, São Paulo and Santa Catarina States) [13]. Supporting the isoenzyme results, analysis of the molecular polymorphism and genetic differentiation of the timeless gene among Brazilian populations of An. cruzii also indicated two cryptic species, one occurring in the north-east (Bahia State) and another in south and south-east Brazil (Espírito Santo, Rio de Janeiro, São Paulo and Santa Catarina States). In addition, the timeless gene sequences also suggested that populations from the south and south-east regions might also constitute different incipient species within this complex in Brazil [14]. Since previous X chromosome analyses suggested the existence of sibling species in these Brazilian regions [9, 10], it would be interesting to analyse the same populations with an X-linked molecular marker to see whether a higher level of differentiation is found.

The gene encoding the NADPH-cytochrome P450 reductase (CPR) has been cloned from several insect species [15‐17]. In Anopheles gambiae, this protein is encoded by a single copy gene located on the X chromosome [17, 18]. Previous studies have shown that the cytochrome P450 gene family, which is involved in metabolic insecticide resistance, requires CPR to function [19, 20]. Additionally, knockdown of CPR expression increases An. gambiae sensitivity to the insecticide permethrin [21]. Another putative function associated with CPR in insects is odorant clearance, since the cpr gene is highly expressed in the antennae of the fruit fly Drosophila melanogaster and more specifically at the base of olfactory sensilla in the moth Mamestra brassicae[16, 22]. Olfactory cues are important environmental stimuli affecting mosquito behaviour, playing significant roles in the location of food sources, mates and oviposition sites [23]. Therefore, genes involved in the regulation of antennal response to pheromones can be potentially important in maintaining sexual isolation between closely related species, and in this case, the CPR would be an interesting molecular marker for population studies of the An. cruzii complex.

In this study, an analysis of intraspecific variability and genetic divergence among five Brazilian populations of An. cruzii was carried out using a fragment of the cpr gene.

The mosquitoes used in this study were females captured at different localities along the Brazilian Atlantic forest and identified on the basis of their morphology according to Consoli and Lourenço-de-Oliveira [4]. A total of 56 individuals were used for the molecular analysis: 14 from Florianópolis, Santa Catarina State (SC) (27°31'S/48°30'W), 12 from Cananéia, São Paulo State (SP) (25°01'S/47°55'W), 12 from Juquitiba, São Paulo State (SP) (23°57'S/47°03'W), 11 from Itatiaia, Rio de Janeiro State (RJ) (22°27'S/44°36'W) and 7 from Santa Teresa, Espírito Santo State (ES) (19°56'S/40°35'W) (Figure 1).

For the isolation of a fragment of the An. cruzii cpr gene, initially a pair of degenerated primers was designed based on conserved regions of the CPR proteins from Drosophila melanogaster, Drosophila pseudoobscura, Musca domestica, Aedes aegypti and An. gambiae (Table 1 and Additional file 1). These primers, named here 5'Cpr01deg and 3'Cpr01deg, were used in PCR with An. cruzii genomic DNA extracted according to Jowett [24]. PCR was carried out with an Eppendorf Mastercycler^® thermocycler using the following conditions: 15 cycles at 94°C for 60 s, 50°C (decreasing 1°C/cycle) for 90 s and 72°C for 60 s, followed by 20 cycles of 94°C for 60 s, 50°C for 90 s and 72°C for 60 s. PCR products were then purified and cloned using Zero Blunt TOPO PCR cloning kit (Invitrogen).

Sequencing of positive clones was carried out in an ABI Prism 3730 DNA sequencer (PDTIS-FIOCRUZ DNA sequencing facility) using the ABI Prism Big Dye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems). The identity of the cloned fragments was confirmed by BlastX analysis using the GenBank [25]. Based on these initial sequences, two new specific primers named 5'cpr01ancruzii and 3'cpr01ancruzii (Table 1 and Additional file 1) were designed to amplify a ~180 bp fragment of the An. cruzii cpr gene from individual mosquitoes of the different localities listed above. This short fragment includes an intron of variable size (see below) and two small segments of the flanking exons (Additional file 1). PCR amplification using the specific primers was carried out for 35 cycles at 94°C for 30 s, 50°C for 60 s and 72°C for 90 s using the proofreading Pfu DNA polymerase (Biotools). PCR fragments were cloned using either Zero Blunt TOPO PCR cloning kit (Invitrogen) or pMOS Blue vector blunt-ended cloning kit (GE Healthcare) and at least eight clones of each mosquito were sequenced.

Sequences were edited and in most cases consensus sequences representing the two alleles were generated. In a number of individuals only one haplotype was observed among the eight sequences and in these cases the mosquitoes were classified as homozygotes. The probability of incorrectly classifying a heterozygote as a homozygote individual with this procedure is less than 1%. Eight homozygotes were found in Florianópolis, four in Cananéia, seven in Juquitiba, three in Itatiaia and two in Santa Teresa. The sequences from homozygote mosquitoes were duplicated prior to analysis. However, the analysis was also carried out without duplicating the homozygote sequences with similar results.

The cpr sequences were aligned using ClustalX software [26] and the population genetics analysis was carried out using DNASP4.0 [27], P_ROS_EQv 2.91 [28] and Arlequin 3.0 [29] softwares. The haplotype network was estimated using TCS1.21 [30].

The X chromosome seems to be enriched in genes that cause reproductive isolation between species in the genus Drosophila[32]. In addition, many sibling species including the An. gambiae complex are outcomes of recent speciation processes associated with paracentric inversions involving this chromosome [33]. The X chromosome banding patterns and inversion frequencies studies of Brazilian south and south-east An. cruzii populations, showed three X chromosomal forms (A, B and C), suggesting a process of incipient speciation [9, 10]. The authors observed that the majority of mosquitoes from Juquitiba population had form A, while form B predominated in Cananéia [9, 10]. In the current study, although there are no fixed differences in the cpr gene between Juquitiba and Cananéia, a moderately high F _ST value was observed. In An. gambiae, cpr is located on the X chromosome. Therefore, if this molecular marker has a similar chromosomal location in An. cruzii, it might be associated with the chromosomal forms described by Ramirez & Dessen [9, 10].

Comparisons of the F _ST values observed with cpr and timeless in all pair-wise comparisons involving the five populations analysed in the current study and in Rona et al[14] show only a partial consistence but this is expected. Wang-Sattler et al[34] demonstrated that the phylogenetic relationships in the An. gambiae complex could vary widely between different genomic regions, thus indicating the mosaic nature of the genome of these species [34].

As mentioned above the cpr gene in An. gambiae is X-linked, while timeless is autosomal. Assuming these two markers have similar locations in An. cruzii, cpr is expected to be under more efficient selection than the timeless, since in species with X/Y sex determination, as An. cruzii, rare recessive mutations are fully expressed in the heterogametic sex, which could lead to 'faster-X evolution' if a large proportion of mutations are fixed by positive selection [35]. If positive selection is more efficient on the X chromosome, one expects it to harbour less variability than the autosomes [36]. The X chromosome is indeed less variable than the autosomes in non-African populations of Drosophila simulans[37]. Comparing timeless and cpr, the first is more polymorphic than the latter, but the latter shows higher differentiation among the southern populations of An. cruzii.

Since An. cruzii is polymorphic for chromosomal inversions and Ramirez & Dessem [9, 10] found evidence for sibling species carrying different X chromosomal forms, another hypothesis that might explain the differences between the two markers is the suppressed-recombination model of speciation proposed by Coluzzi [33, 38].

Analysis of the molecular polymorphism and genetic differentiation of the timeless gene among Brazilian populations of An. cruzii suggested that the population from Itatiaia (Rio de Janeiro State) is in a process of differentiation and incipient speciation [14]. High F _ST values between Itatiaia (Rio de Janeiro State) and the other populations from south and south-east Brazil was reported here. In addition, comparison of Itatiaia with other populations revealed some fixed differences and only a few shared polymorphisms. Moreover, the haplotype network shows that Itatiaia is clearly separated in an isolated group (Figure 3). These results, therefore, suggest that this population represents a different species in the An. cruzii complex.

Preliminary evidence was also presented here that raised the possibility of the existence of two different sympatric incipient species in Itatiaia. This is based on the analysis of the genetic differentiation of the cpr gene and a reanalysis of the recently published timeless data [14]. Although a putative heterozygote was found in cpr analysis considering the AG repeats and shared polymorphisms were observed in timeless, high F _ST values were detected between Itatiaia A and Itatiaia B in these two molecular markers, as well as fixed differences which seem to indicate that these two groups might represent different incipient species. Inspection of the neighbour-joining tree presented in the timeless study [14] reveals that the individuals classified here as Itatiaia A are clearly isolated in a separated branch. However, the individuals classified as Itatiaia B are mixed with the other individuals from south and south-east populations. In that study, only one putative heterozygote was found (Ita01) carrying alleles of the two Itatiaia groups. Unfortunately this DNA sample was lost and therefore it was not possible to analyse the cpr gene of this individual mosquito. The sample sizes available for the two Itatiaia groups are quite small and further work is clearly needed to determined beyond any doubt that two sympatric incipient sibling species exist in this locality but the results presented here seems to indicate that might be the case.

Analysis of a number of other molecular markers will allow a more precise estimate of the Itatiaia population differentiation and might provide a more complete representation of the divergence history of this species complex.

Evidence was presented here suggesting the existence of at least one new sibling species within the Anopheles (Kerteszia) cruzii complex in Itatiaia, south-east Brazil, a finding that supports a previous timeless gene study. In addition, according to cpr and timeless gene analyses, the Itatiaia sample might be in fact composed by two sympatric incipient species, named here Itatiaia A and Itatiaia B.