Tanzania Journal of Health Research Health User's Trust Fund (HRUTF) ISSN: 1821-6404 Vol. 10, Num. 2, 2008, pp. 99-102

Tanzania Journal of Health Research, Vol. 10, No. 2, April, 2008, pp. 99-102

Morphometric studies on Culex quinquefasciatus and Mansonia africana (Diptera: Culicidae) in Abeokuta, south-western Nigeria

M.A. Adeleke^*, C.F. Mafiana, A.B. Idowu, M.F. Adekunle and B.M. Dansu

Department of Biological Sciences, University of Agriculture, P.M.B 2240, Abeokuta, Ogun State, Nigeria
Department of Forestry and Wild life Management, University of Agriculture, Abeokuta, Ogun State, Nigeria
Department of Statistics, University of Agriculture, Abeokuta, Ogun State, Nigeria
^* Correspondence: Monsuru Adeleke; E-mail: healthbayom@yahoo.com

Received 15 November 2007 Revised 13 March 2008 Accepted 15 March 2008

Code Number: th08018

Abstract

Some of the important mosquito species are generally sympatric in terms of their geographical distributions, and are difficult to identify based solely on examination of adult females using the available morphological keys. In this study, adult mosquitoes were collected indoors in Abeokuta in south-western Nigeria between August 2005 and July 2006 using Center for Disease Control light traps. The objective was to investigate possible morphological differences in populations of Culex quinquefasciatus and Mansonia africana. Six morphological characters namely, wing length, antennal length, proboscis length, foreleg length, mid leg length and hind leg length were measured in the two species. A total of 868 Cx quinquefasciatus and 962 M. africana were collected during the study period. The mean length was observed to be higher in most characters during the wet season than the dry season but the variation was not statistically signifi cant (P> 0.005). In M. africana, the antennal length, proboscis length, foreleg length and midleg length showed one peak each. The wing length exhibited three peaks while hind leg length showed two peaks. For Cx quinquefasciatus, the antennal length, proboscis length, foreleg length, mid leg length and hind leg length indicated one peak. The wing length however showed three peaks. Each of the peaks observed in wing length and hind leg length of M. africana and wing length of Cx quinquefasciatus was assumed to be a specific population. The coefficient of differences (CDs) for each population indicated the presence of three populations in M. africana (CD > 1.28). However, the hind leg length showed that the two populations were the same (CD< 1.28). Three populations were also obtained from CD for wing leg length of Cx quinquefasciatus (CD>1.28). Other characters indicated the presence of only one population. The results therefore underscore the need to investigate the status of the two species in relation to species complex.

Keywords: Morphometric analysis, Culex quinquefasciatus, Mansonia africana, Nigeria

Introduction

Some of the mosquito vectors of public health importance are generally sympatric in terms of their geographical distributions. Such vectors generally present high degree of morphological similarity at adult stage and many times difficult to identify based solely on examination of adult females using the available morphological keys (Calle et al., 2002). This problem is common with Anopheles gambiae and An. funestus species complexes. In epidemiological studies (on disease transmission or evaluation of control measures) it is necessary to identify the species of adult females which are found near humans. Traditionally, morphometric studies have contributed substantially in resolving taxonomic problems in mosquito identification (Delgado & Rubio-Palis, 1993; Petraca et al., 1998; Calle et al., 2002).

Culex quinquefasciatus and Mansonia africana are among the most abundant mosquitoes in Africa. The two species are also vectors of a number of human and animal pathogens and parasites including viruses and nematodes. Cx quinquefasciatus is one of the major vectors of Wuchereria bancrofti infection in the world (White, 1971; Subra, 1981) while M. africana is known to transmit Rift Valley fever (Meegan & Bailey, 1988; Fontenille et al., 1998), and yellow fever.

Observations had long been made on the existence of species complex in many dipteran insects (Gillett, 1972). These observations were based on inconsistent behavioural habits of many species in different geographical zones. Most importantly, member of the family Culicidae (mosquitoes) have been known to consist of many closely related siblings (Lehane, 1991). Most of these siblings had however been separated either through molecular characterization or multivariate analysis (morphometrics) (Adesiyan et al., 1998). The use of morphometric technique relies mainly on variation in size and shape of the insects (Daly, 1985).

In tropical region of the world, Culex pipiens complex, a group in which Cx quinquefasciatus belongs, has been a subject of many taxonomic and genetic studies after the discovery of closely related interbreed siblings in the complex (Knight, 1978; Pryor & Daly, 1991). Though, attempts had been made to separate siblings of this complex by measuring the distance between the tip of the genital arm to its intersection with the dorsal (Sundararaman, 1949; Barr & Kart-man, 1951; Pryor & Daly, 1991), there is paucity of information on the use of other morphological characters in discriminating siblings of this species. Apart from the differences in ecotypes which could render the result from one geographical zone inapplicable in others, morphometric techniques have not been widely applied on Cx quinquefasciatus in Africa. However, no attempts have been made in investigating M. africana species status. It is against this background that the present work was designed to investigate the possible morphological differences in Cx quinquefasciatus and M. africana in Abeokuta, south-western Nigeria.

Materials and Methods

The research was conducted in Abeokuta Metropolis located on approximately latitude 7^o10N and longitude 3^o21E in the transitional zone between tropical rainforest and derived savannah zone in the southwestern, Nigeria. Abeokuta usually experiences two seasons; the dry season (November to March) and the wet season (April to October). Five stratifi ed locations were selected for the study namely Ago-Ika, Ijaye, Kugba, Ibara and Obantoko.

Adult mosquitoes were collected in three randomly selected houses in each of the study locations once a week using Center for Disease Control (CDC) light traps (Model 512, J.W. Hock Ltd, Gainesville, Florida, USA), between August 2005 and July, 2006. Each trap was suspended from the roof about 1.8m above the floor and 0.5m from a bed occupied by an adult sleeper. The trap was operated with 6.0 volt rechargeable battery every week. The sleeper in each room was instructed to switch on the trap at 20:00 hr and switch it off at 5:00 hr after the neck of the collection bag has been properly tied. All mosquitoes collected in the traps were removed and kept in labelled EDTA bottles for laboratory analysis. Cx quinquefasciatus and M. africana were sorted out of the other mosquitoes using keys described by Gillett (1972). Six characters, antennal length, proboscis length, wing length, foreleg length, mid leg length and hind leg length were carefully detached and measured in each mosquito using calibrated microscope.

Statistical analysis

Monthly means of all measured characters in each of the two species were subjected to regression analysis. Co-efficient of difference (CD) was calculated for each character having at least two peaks after the frequency distribution of the grouped data has been plotted. The CD was calculated according to Mayr (1969a)

Where

CD = Co-efficient of difference
M_b = Population with the large mean
M_a = Population with the smaller mean
SD_b = Standard deviation of the population with the larger mean
SD_a = Standard deviation of the population with the smaller mean.

Results

A total of 868 Cx quinquefasciatus and 962 M. africana were collected during the study period. The results of morphometric analysis on Cx quinquefasciatus and M. africana revealed that there were variations in the characters. The mean length was observed to be higher in most characters during the wet season than the dry season but the variation was not statistically signifi cant (P> 0.005) (Table 1).

In M. africana, the antennal length, proboscis length, foreleg length and midleg length showed one peak each. The wing length exhibited three peaks while hind leg length showed two peaks. For Cx quinquefasciatus, the antennal length, proboscis length, foreleg length, mid leg length and hind leg length indicated one peak. The wing length however showed three peaks. Each of the peaks observed in wing length and hind leg length of M. africana and wing length of Cx. quinquefasciatus was assumed to be a specific population, and in order to determine the uniqueness of each of the populations, coefficient of differences (CDs) were calculated for each population. The results obtained from CD also indicate the presence of three populations in M. africana (CD > 1.28) but the hind leg length showed that two populations were the same (CD< 1.28) (Table 2). Three populations were also obtained from CD for wing leg length of Cx quinquefasciatus (CD>1.28) (Table 2).

Discussion

Morphological variation was observed in the measured characters in Cx quinquefasciatus and M. africana which was not season specific. The variation may be associated with the physiological conditions of individual or may be genetically controlled. According to Mayr (1969b), most of the morphological variation is clinal and thus, the variation observed in some characters as revealed by CD may be clinal.

The indication of three populations by wing length of Cx quinquefasciatus may probably be possible. Until recently, Cx pipiens pipiens, Cx pipiens quinquefasciatus and Cx. pipiens fatigans have been regarded as subspecies of Cx pipiens complex and they have been taken as three taxonomically distinct populations (Jakob et al., 1980; Pryor & Daly, 1991). Cx pipiens complex were later observed to consist of hybrid and intermediate forms of Cx. pipiens pipiens and Cx pipiens quinquefasciatus as reported in various morphometric studies, genetic and molecular assays using polymerase chain reaction (Cornell et al. 2003; Smith & Fonseca, 2004). The wing length could also be another variation in their morphological composition and may be used in sorting members of the complex.

The wing length of M. africana indicated three probable populations but the hind leg length showed that two populations are the same. Though, the complex of this species has not been reported in any part of the world, its irregular biting behaviour (anthropophilic and zoophilic) in different geographical zones (Gillett, 1972; Amusan, 2004) underscores the need to investigate its species status.

Further studies are therefore recommended on molecular and morphometric analysis of these two species, most importantly on M. africana, so as to shed light on its species status.

Acknowledgements

The authors are grateful to Mr. Adeyi Akindele, Drs. J.C. Anosike and S.O. Sam-Wobo for their useful suggestions and advise during the preparation of this manuscript. We are also grateful to the late Dr. A.A.S. Amusan for his technical assistance during the course of this study.

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