Differentiation and genetic variability in natural populations of Anopheles (N.) triannulatus (Neiva& Pinto, 1922) of Brazilian Amazonia

Diferenciação e variabilidade genética em populações naturais de Anopheles (N.) triannulatus (Neiva & Pinto, 1922) da Amazônia brasileira

Differentiation and genetic variability in natural populations of Anopheles (N.) triannulatus (Neiva& Pinto, 1922) of Brazilian Amazonia

Diferenciação e variabilidade genética em populações naturais de Anopheles (N.) triannulatus (Neiva & Pinto, 1922) da Amazônia brasileira

Santos, J. M. M.; Maia, J. F.; Tadei, W. P.

Instituto Nacional de Pesquisas da Amazônia, Av. André Araújo, 2936, Petrópolis, C.P. 478, CEP 69011-970, Manaus, AM, Brazil

^{Correspondence}

ABSTRACT

Populations of Anopheles triannulatus from Macapá (AP), Aripuanã (MT), Ji-Paraná (RO), and Manaus-Janauari Lake (AM) were studied using 16 enzymatic loci. The results of the isozyme analysis showed that the population of Macapá presented higher polymorphism (56.3%). The lowest variability was observed in the population of Manaus (p = 25.0; Ho = 0.077 ± 0.046). The results of Wright's F statistics showed unbalance due to excess of homozygotes (F_is > F_st), denoting a certain intrapopulational differentiation. Although the populations are genetically very close (D = 0.003 - 0.052), the dendrogram separates the populations in two groups: Macapá separated from that of Manaus, Ji-Paraná, and Aripuanã. This result may suggest a reduction in the genic flow, which possibly had some influence in the substructuration of the populations.

Key words:Anopheles triannulatus, isozymes, genetic of population, malaria.

RESUMO

Populações de Anopheles triannulatus procedentes de Macapá (AP), Aripuanã (MT), Ji-Paraná (RO) e Manaus-Lago Janauari (AM) foram estudadas utilizando-se 16 locos enzimáticos. Os resultados mostraram maior polimorfismo (56,3%) na população de Macapá. A menor variabilidade foi verificada na população de Manaus (p = 25,0; Ho = 0,077 ± 0,046). Os resultados das estatísticas F de Wright mostraram desequilíbrio decorrente de excesso de homozigotos (F_is > F_st), denotando certa diferenciação intrapopulacional. Embora as populações sejam geneticamente muito próximas (D = 0,003 - 0,052), o dendrograma separa as populações em dois grupos: Macapá separado de Manaus, Ji-Paraná e Aripuanã. Isto pode ser indicativo de redução no fluxo gênico, que possivelmente influenciou a subestruturação das populações.

Palavras-chave:Anopheles triannulatus, isoenzimas, genética de população, malária.

INTRODUCTION

Anopheles triannulatus of the subgenus Nyssorhynchus presents wide distribution in South America and part of Central America. In South American it occurs mainly to the east of the Andes, in the Guyanas, Colombia, Venezuela, Ecuador, Peru, Bolivia, Paraguay, Argentina, and the whole Brazilian territory (Lane, 1953; Forattini, 1962; Faran, 1980). It shows zoophilic, exophilic, and crepuscular behavior for most of the Brazilian populations, placing this species among the secondary or potential vectors of malaria (Forattini, 1962; Tadei et al., 1983, 1988a, 1998; Deane, 1988).

This species is common in forests, where it shows hematophagic behavior in the upper strata of the vegetation (Deane et al., 1971; Lourenço-de-Oliveira, 1993). However, it has also been found attacking man in the peridomicile as well as inside houses (Gabaldon, 1949; Charlwood & Wilkes, 1981; Tadei et al., 1983, 1988a; Lourenço-de-Oliveira et al., 1989; Rúbio-Palis, 1994). Nevertheless, its role as a vector of human malaria is still controversial. In Venezuela, A. triannulatus was identified as a possible vector during a malaria outbreak (Benarroch, 1931) and found to be naturally infected with plasmodian oocytes by Gabaldon & Cova Garcia (1946). Faran & Linthicum (1981) stated that A. triannulatus has no importance in malaria transmission in a large part of its occurrence area.

However, natural infections in this anopheline, detected by radioimmunoassay and immunoenzimatic tests with monoclonal antibodies for research of the protein CS in material from Peru, were reported by Hayes et al. (1987). The same is true for certain areas of Brazilian Amazonia, where A. darlingi is responsible for the transmission and continuation of malaria (Arruda et al., 1986; Tadei et al., 1988b; Oliveira-Ferreira et al., 1990). Some authors believe that A. triannulatus presents epidemiological importance in relation to malaria when it occurs in areas of high population density (Hill, 1934; Charlwood & Wilkes, 1981). This species was separated from the subgroup Oswaldoi of the group Oswaldoi by Faran (1980) and, due to differences in taxonomic characters, placed in the monotypic subgroup Triannulatus.

Morphological, behavioral, and epidemiological differences have been observed, complicating the taxonomy states of this species, which is either considered to be a highly polymorphic single species or separated into two subspecies: A. triannulatus triannulatus and A. triannulatus davisi (Galvão & Lane, 1941; Forattini 1962; Nascimento, 1995). In spite of this variation and controversy regarding the taxonomic status of the species, it has been the object of few genetic studies, the first of which were carried out by Conn (1991). Whose polythenic chromossome analysis of samples from Venezuela showed some associations of the chromossomes arms which differentiate it in this particular from those of the other members of the subgenus Nyssorhynchus. Santos et al. (1992) studied a population of A. triannulatus from Balbina (AM) based on five esterase loci, all of them polymorphic. The values of 2 for the allelic frequencies were highly significant for all loci in that population.

Given these differences and the possibility of A. triannulatus being a complex of sibling species, populations from four places in Brazilian Amazonia were studied, with the aim of contributing towards elucidation of these taxonomic problems, and describing the genetic structure of the populations regarding their enzymatic variability.

MATERIAL AND METHODS

The mosquitoes were obtained from natural populations of Macapá (00º02'19.8''N, 51º03'40.7''W), Amapá State (AP); Aripuanã (10º10'00''S, 59º27'34''W), Mato Grosso State (MT); Ji-Paraná (10º53'07''S, 61º57'06''W), Rondônia State (RO); and the Janauari Lake Manaus (03º08'00.5''S, 60º00'34.7''W), Amazonas State (AM) (Fig. 1).

Adult mosquitoes were caught in animal enclosures next to human dwellings and in peridomestic sites between 18:00 and 22:00 h. After the collections, females were allowed to lay eggs separately and, soon after the egg hatching, larvae were maintained until becoming adults. Five fourth-instar progeny of each field-collected female were used for each enzymatic system, except for the Ji-Paraná population (three individuals on average). The adults were identified soon after their emergence and kept at -70ºC or deposited in the laboratory entomological collection. For identifying the mosquitoes, the keys of Gorham et al. (1967) and Consoli & Lourenço-de-Oliveira (1994) were used. The larvae were maintained in the laboratory according to methods described by Santos et al. (1981).

Sixteen enzymatic loci were analyzed: EST1, EST2, EST4, LAP2, LAP5, PGI, XDH, HK1, HK2, HK3, HK4, IDH, MDH, ME, 6-PGDH, and PGM in starch gel (12.5%) and starch-agarose (2% and 1%, respectively) electrophoresis following the method described by Steiner & Joslyn (1979), Lima & Contel (1990) and Santos et al. (1996).

Allelic frequencies were estimated directly from the data. Polymorphic loci ratio (P), heterozygosities found (Ho) and expected (He), and Wright's coefficients were estimated in each population by using the Biosys (Swofford & Selander, 1981) program. The dendrogram was constructed employing the UPGMA method (Nei, 1978).

RESULTS

Ten of the 16 loci analysed showed genetic variability in the four populations: EST1, EST2, EST4, LAP2, LAP5, IDH, MDH, ME, 6-PGDH, and PGM. The PGI, XDH, HEX1, HEX2, HEX3, and HEX4 loci were monomorphic in all populations. The LAP2 and ME loci were polymorphic only in the Macapá population, while EST1, LAP5, and IDH showed variation only in the Manaus population. The 6-PGDH locus showed variation only in the Aripuanã population (Table 1). Alleles B (EST1, LAP2, and LAP5), E (EST4), and C (IDH1, MDH, 6-PGDH, and PGM) presented the lowest frequencies in the four populations. Hardy-Weinberg's test indicated that most of the loci are in balance in all the populations.

Table 2 shows a genetic variability estimate of the studied populations. The Macapá population is the most polymorphic (p = 0.56) and its observed (Ho = 0.133 ± 0.054) and expected (He = 0.174 ± 0.055) average intralocus heterozygosity was higher. Lowest variability was observed in the Manaus population, with lower polymorphism (p = 0.25) and heterozygosity (Ho = 0.077 ± 0.046; He = 0.076 ± 0.046) and fewer alleles per locus (1.4 ± 0.2). In spite of the low variability detected in the Manaus population, the values of c² were not significant at the 5% level in three of the four polymorphic loci. The populations of Macapá and Manaus showed excess heterozygotes in most of the loci, while excess of homozygotes was observed in the Ji-Paraná and Aripuanã populations.

Genetic structure analysis of the populations by Wright's F statistics showed an average F_is value higher than F_st (F_is = 189 > F_st = 0.110). Higher F_is values in relation to F_st were observed in all loci in the four populations (Table 3). This result may be largely due to the LAP2 and ME loci which were polymorphic only in the Macapá population and, thus have contributed to the high F_is values (0.525 and 0.968, respectively), indicating high genetic substructuration. The genetic distance values were low among the populations, showing little interpopulational differentiation (Table 4). The largest distance was found between Macapá and Manaus and the smallest between Aripuanã and Ji-Paraná. The genetic distance dendrogram separates the populations in three clusters. The first separates Manaus, Ji-Paraná, and Aripuanã from Macapá, the latter being isolated from the others in a single cluster. The second cluster separates Manaus from Ji-Paraná and Aripunã.

DISCUSSION

The allelic frequencies were not uniform in the different loci, as indicated in the Hardy-Weinberg test of which are in balance in all populations. One of the most important implications in Hardy-Weinberg's balance is that when an allele is rare, many of the individuals can be heterozygotes (Hartl, 1981). The heterozygote excess observed in most of the loci in the Macapá and Manaus populations may be an indication of greater adaptability of these genotypes in response to environmental alterations. In spite of these two populations occurring in different geographical areas, both were caught on cattle in enclosures close to homes and river margins greatly influenced by the anthropic and natural (flood/low water regime environment). Nevo (1978) suggests that the heterozygote excess may also be due to epistatic interactions or to frequency-dependent selection. Heterozygote excess was verified by Scarpassa et al. (1999), in an MDH locus of A. nuneztovari populations of Tibu and Sitronela, in Colombia. However, heterozygote excess was detected in the Ji-Paraná and Aripuanã populations, and Hardy-Weinberg's test showed balance for most loci in these populations. The same results have frequently been reported for other mosquito populations (Santos et al., 1985; Van Driel et al., 1987; Hii et al., 1991; Scarpassa, 1988) and other insects (Sturgeon & Milton, 1986; Crouau-Roy, 1988). According to these authors, the deficiencies were attributable to differential genotypical selection, inbreeding, null alleles, a bottleneck effect, and the intrinsic population structure. In A. nuneztovari of Tucurui, Scarpassa (1988) attributed heterozygote deficiency in three esterase loci to ecological alterations in that area which possibly affected genotypical frequencies, resulting in different adaptive values for the homozygote classes.

Average heterozygosity observed in the populations of A. triannulatus (0.077-0.133) is similar to those obtained for other anopheline species of the subgenus Nyssorhynchus (Steiner et al., 1982), and very close to the populations of A. nuneztovari of Brazil and Colombia (0.078-0.122) analyzed by Scarpassa et al. (1999), as well as that of Andean Venezuela (0.086-0.0118) described by Fritz et al. (1995). However, these results differ from those obtained by Santos et al. (1999) in populations of A. darlingi of Brazilian Amazonia (0.236-0.432).

Unbalance as a result of homozygote excess was observed in lineages of A. stephensi (Van Driel et al., 1987), in which the value of F_is is higher than that of F_st for most loci analyzed. The average fixation index among the loci was low (F_it = 0.107). However, the GPDH locus of A. stephensi, and LAP2 and ME of A. triannulatus showed high fixation indexes (F_it = 0.831, 0.584, and 0.982, respectively). A similar result was observed by Santos et al. (1999) in the locus AO of populations of A. darlingi of Brazilian Amazonia (0.663) and by Scarpassa et al. (1999), in the loci a-GPDH, LAP1, and PGI (0.910, 0.569, and 0.799, respectively) of populations of A. nuneztovari of Brazil and Colombia. For A. triannulatus, the fixation index (F_is > F_st) may indicate some kind of isolation existing in some of the analyzed loci which interferes in random crossing. Genetic divergence among populations can be observed when a certain degree of reproductive isolation has already been established in the populations, as described by Narang et al. (1990).

The allelic frequency variability in A. triannulatus populations based on the fixation data is of intrapopulational origin and result in a certain differentiation among these populations. This differentiation may possibly be produced by homozygote excess in some loci and/or in the LAP2 and ME loci, which were polymorphic only in the Macapá population.

The distance values found among the four populations are in the range proposed for anophelines intra-specific variation (Bullini & Coluzzi, 1982), indicating genetic homogeneity on the macro-geographical scale, in spite of intrapopulational geographical differentiation. The small genetic divergence (D = 0.003-0.052) observed in A. triannulatus was similar to that shown in the data obtained by Santos et al. (1999) among A. darlingi populations of (D = 0.010-0.024) and by Scarpassa et al. (1999) among A. nuneztovari populations of Brazil (0.001-0.032).

Comparing these distance values and genetic similarity with those obtained for anopheline species, a similarity is detected, except for interspecific cases. For example, for the complex A. marshalli, the isoenzime analysis showed little divergence, with a genetic similarity of 0.803-0.972 (Lambert, 1983).

The phenogram (Fig. 2) shows that A. triannulatus populations are genetically separated in two groups: the Macapá population separated from a cluster of the Manaus, Ji-Paraná, and Aripuanã populations, and these separated in two clusters. This result may indicate a genic flow reduction, based on populations substructuration. Additional molecular studies will be necessary to better define A. triannulatus genetic structure and taxonomic status.

Acknowledgments — The authors are grateful to Dr. Raul Guerra de Queiroz for help with the English version of the manuscript. The research was financially supported in part by the Pilot Program for the Protection of Brazilian Rainforest/MCT- Subprogram C & T/PPDG-7 and Institutional Research Program (PPI-3130).

Received February 19, 2002 - Accepted April 25, 2003 - Distributed May 31, 2004

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Publication Dates

• Publication in this collection
  20 Aug 2004
• Date of issue
  May 2004

History

• Accepted
  25 Apr 2003
• Received
  19 Feb 2002