Memórias do Instituto Oswaldo Cruz Fundação Oswaldo Cruz, Fiocruz ISSN: 1678-8060 EISSN: 1678-8060 Vol. 96, Num. 2, 2001, pp. 197-204

Mem Inst Oswaldo Cruz, Rio de Janeiro, Vol. 96(2) Feb. 2001, pp. 197-204

Ootaxonomic Investigation of Five Lutzomyia Species (Diptera, Psychodidae) from Venezuela

AM Fausto, MD Feliciangeli*, M Maroli**, M Mazzini

Dipartimento di Scienze Ambientali, Università della Tuscia, Via San Camillo de Lellis, 01100 Viterbo, Italy *Escuela de Medicina, Facultad de Ciencias de la Salud, Universidad de Carabobo, Núcleo Aragua, Maracay, Venezuela **Laboratorio di Parassitologia, Istituto Superiore di Sanità, Roma, Italy

Corresponding author. Fax:+39-0761-357114. E-mail: fausto@unitus.it

Received 8 February 2000
Accepted 15 August 2000

Code Number: oc01028

The eggshell fine structure of five sand fly species from Venezuela belonging to the genus Lutzomyia (L. migonei, L. ovallesi, L. absonodonta, L. gomezi and L. panamensis) was examined by scanning electron microscopy. The chorionic sculpturing of L. migonei, L. ovallesi, L. absonodonta and L. gomezi was characterized by series of columns arranged in palisade to form sinuous ridges. In inter-ridge areas, the basal layer was covered with fibrous material. The outer chorion of L. panamensis had a pattern known as "mountain- or volcano-like". The morphology of the posterior pole and aeropyle had a common structure in the five species, with some species-specific characters. The eggshell features of the five species are compared with those of other phlebotomine sand flies.

Key words: phlebotomine sand fly - ootaxonomy - eggshell - aeropyle - fine structure - scanning electron microscopy

Sand fly taxonomy was declared to be "an indispensable basis for every work on sand fly biology and on their vector role" by the World Health Organization (WHO 1977). However, the taxonomic status of phlebotomines is much more obscure than that of other insects of medical significance, such as mosquitoes (Lane 1986), although many attempts have been made to find unequivocal characters to distinguish morphologically similar species and to propose stable classifications (Lewis et al. 1977, Artemiev 1991, Ashford 1991).

Scanning electron microscope (SEM) studies of eggshell morphology have shown that differences between related species of insects are reliable taxonomic markers (Hinton 1981, Mazzini 1987, Mazzini et al. 1993a). Ootaxonomy of various families of dipterans is in an advanced stage (for a review, see Margaritis & Mazzini 1998). With regard to sand flies, studies on eggshell morphology have been delayed by the difficulty of finding eggs in nature and of breeding certain species. The eggs of Old World sand flies have only been studied for a limited number of species (Irungo et al. 1985, Lane & El Sawaf 1986, Gebre-Michael & Lane 1991, Fausto et al. 1991, 1992, 1993, Rogo et al. 1992, Ghosh & Mukhopadhway 1996). These studies suggest that differences in egg sculpture of Sergentomyia and Phlebotomus species are sufficient for separation of the two genera, hitherto based on other taxonomic characters (Perfil'iev 1966). Ootaxonomic investigations have been carried out in many Neotropical species (about 10% of the about 400 known species) demonstrating the usefulness of chorionic ultrastructure in separating closely related species (Ward & Ready 1975, Zimmerman et al. 1977, Endris et al. 1987, Feliciangeli et al. 1993, Sierra et al. 1995, Perez & Ogusuku 1997).

In the present study, the eggshell fine structure of five sand fly species belonging to the genus Lutzomyia - L. (Psychodopygus) panamensis, L. (Lutzomyia) gomezi, L. (Species Group Verrucarum) ovallesi, L. (Species Group Migonei) migonei, and L. (Micropygomyia) absonodonta - from Venezuela was examined by SEM. The first four species are known to be anthropophilic and have also been demonstrated or incriminated as vectors of Leishmania spp. in different Latin American countries. L. panamensis is regarded as a primary vector of cutaneous leishmaniasis (CL) in Venezuela (Rodriguez et al. 1999) and as a secondary vector in Panama, Guatemala and Colombia (Desjeux 1991). L. gomezi was reported to be naturally infected with Le. braziliensis in Panama (Johnson et al. 1993), Colombia (Young et al. 1987), Ecuador (Gomez & Hashiguchi 1987) and Venezuela (Feliciangeli 1991, Feliciangeli et al. 1994). L. ovallesi is recognized as a vector of CL in Venezuela (Feliciangeli 1991, Feliciangeli et al. 1994) and Guatemala (Rowton et al. 1992). It was found to be infected with a variant of Le. panamensis/guyanensis in Venezuela (Bonfante-Garrido et al. 1991) and with unidentified flagellates in Belize, Panama (Williams 1970) and Colombia (Young et al. 1987). L. migonei is a suspected vector of Le. braziliensis in the State of Ceará, Brazil (Azevedo et al. 1990). The medical significance of L. absonodonta is unknown. Closely allied species in the subgenus Micro-pygomyia are known to feed on lizards.

The chorionic patterns of the five species and other species belonging to different genera of Phlebotominae were compared.

MATERIALS AND METHODS

Newly laid eggs were obtained from naturally blood-engorged sand flies of the following species: L. (Species Group Migonei) migonei, L. (Species Group Verrucarum) ovallesi, L. (Micropygomyia) absonodonta, L. (Lutzomyia) gomezi, L. (Psychodopygus) panamensis.

For scanning electron microscopy, the eggs were fixed for 1 h at 4°C in 4% paraformaldehyde and 5% glutaraldehyde in 0.1 M cacodylate buffer at pH 7.2 (Karnovsky 1965), then rinsed overnight in cacodylate buffer, post-fixed in 1% osmium tetroxide for 1 h and dehydrated in a graded ethanol series. The material was dried by the critical point method using liquid CO₂ in a Balzers CPD 020 apparatus, attached to specimen holders, coated with gold in a Balzers Union MED 010 evaporator and observed with a 5200 Jeol JSM electron microscope.

RESULTS

Common characters of all eggs are elongated, cigar-like form with one side slightly flattened and both poles rounded (Figs 1, 5, 9, 14).

The five species had specific chorionic sculpturing which enabled the species to be distiguished from each other.

L. migonei (Figs 1-4) - The egg had a median width of about 80 µm and a length of about 280 µm (Fig. 1). The eggshell surface was characterised by polygonal design, consisting of longitudinal ridges united by cross-ridges, which defined irregular rectangular areas with a major side parallel to the longitudinal axis of the egg (Fig. 3). Each ridge consisted of a single series of cylindrical columns, about 2 µm high, united at the top (Figs 3-4). In inter-ridge areas, the basal layer was covered in uniformly distributed fibrous material (Figs 3-4).

The posterior pole was delimited by the terminal parts of a few chorionic ridges. Marked protuberances of different size were disposed around the aeropylar openings (Fig. 2).

L. ovallesi (Figs 5-8) - The eggs were about 270 µm in length and 80 µm in width (Fig. 5) with a complete basal layer covered with a compact coat of fine fibrous material (Figs 6, 7). The outer chorion had a series of sinuous longitudinal ridges with a few cross-ridges defining rectangular areas of different size (Fig. 6). The ridges consisted of a double series of columns (high about 0.5 µm) linked at the top (Fig. 7). Some of the chorionic ridges extended as far as the polar regions.

The posterior pole consisted of a circular area delimited by the end part of chorionic ridges and divided into two semicircular areas by two short transverse ridges (Fig. 8). Each area had an aeropylar opening and several small conical protrusions.

L. absonodonta (Figs 9-11) - The eggs were about 300 µm long and 75 µm wide (Fig. 9). The chorionic sculpture consisted of longitudinal columnar ridges defining elliptical areas of about 35-40 µm in length (Fig. 10). Each area was crossed by transverse fine ridges to form irregular quadrilaterals. As a result the egg surface had a reticular pattern. The basal layer of chorion between the ridges was covered in coarsely arranged fibrous material, that formed minute microvilli (Fig. 10). The borders of the posterior pole region were not well defined. This area was covered by several protuberances randomly distributed around the two aeropylar openings (Fig. 11).

L. gomezi (Figs 12-13) - The surface sculpturing of eggs from Venezuelan females had a pattern of polygons formed by intersecting ridges consisting of columns arranged in palisade completely united at the top (Fig. 12). The areas enclosed by the ridges were four-sided with rounded corners and sides measuring about 15-20 µm (Fig. 12). The basal layer was covered with regularly arranged fibrous material. The posterior pole region was devoid of the structures characterising the rest of the egg surface and was delimited by the ends of the chorionic ridges and irregular circular ridges (Fig. 13). Two non-columnar ridges partially divided the polar region into two semicircular areas in which small protrusions were observed beside each aeropylar opening.

L. panamensis (Figs 14-17) - The egg had a median width of about 100 µm and a length of about 340 µm (Fig. 14). The chorionic pattern was different from that of the other species, consisting of a uniform layer of numerous and tall mountains bearing volcano-like structures regularly disposed (Fig. 15). These structures showed prominent irregular edges, enclosing a central depression in which holes of different sizes were visible (Fig. 16). The posterior pole was surrounded by a circle of short non-columnar ridges. The aeropylar openings were surrounded by small uneven ridges (Fig. 17).

DISCUSSION

The eggshell sculptures of the five species showed species-specific characters, useful for species identification. However, they had some features in common with eggs of other New and Old World sand fly species. Morphological categories, based on the chorionic patterns of Lutzomyia eggs, have been proposed by different authors. Ward and Ready (1975) proposed three categories: "volcano-like or mountain-like", "polygonal", and "parallel ridging". The latter was divided by Endris et al. (1987) into "connected ridges" and "unconnected ridges". With the recent discovery of other chorionic patterns, three new categories were added: "elliptical" (Feliciangeli et al. 1993), "verrucose" and "disperse" (Pérez & Ogusuku 1997). With regard to chorionic patterns described in the Old World species, Gebre-Michael and Lane (1991) divided the category "unconnected ridges" into the groups: "fragmented chained" and "complete chained". The latter was defined as "reticular" by Fausto et al. (1992).

The five species described in this paper can be grouped in the following categories: L. migonei "connected parallel ridges"; L. ovallesi "connected parallel ridges" (with few connections); L. absonodonta "reticular"; L. gomezi "polygonal"; L. panamensis "volcano-like or mountain-like".

Most of patterns (L. migonei, L. ovallesi, L. absonodonta, and L. gomezi) have prominent ridges consisting of one or more series of columns. These patterns, common in New World species, are the only ones present in Old World species belonging to genera Phlebotomus and Ser-gentomyia (Fausto et al. 1992, 1993). However, Phlebotomus eggs show great variability in the arrangement of the columns, and the eggs of Sergentomyia species are morphologically uniform, with only a polygonal pattern and none of the fibrous material usually covers the basal layer between the ridges. These results are in line with comparative spermatology data, suggesting that the genus Sergentomyia followed a different evolutionary path to Phlebotomus and Lutzomyia (Dallai et al. 1984, Mazzini et al. 1993b, Fausto et al. 1995).

The "volcano or mountain-like" pattern described in L. panamensis eggs has only been observed in species of the Lutzomyia genus. This particular pattern may have evolved in response to a damp microhabitat, i.e. swamp forest, in which these species are abundant (Ward & Ready 1975). As shown in Fig. 16, the prominent "volcano-like" structures protecting the aeropylar openings, are probably the basis for a well developed plastron and reflect its habitat preference. The chorionic sculpturing of L. panamensis from Panama (Zimmerman et al. 1977) is very similar to that of L. panamensis from Venezuela, whereas two different chorionic patterns have been described for L. gomezi from Panama (Zimmerman et al. 1977) and Colombia (Sierra et al. 1995) (elongated hexagonal polygons) and Brazil (Ward & Ready 1975) (regular pentagonal polygons). We found that eggshell sculpture of the Venezuelan specimens was of the Brazilian type.

Almost nothing is known about the breeding sites of the present and previous species described, and there is not yet enough data to attempt a taxonomic and evolutionary interpretation. However, the similar exochorion pattern in L. gomezi from Panama and Colombia and the other pattern shared by L. gomezi from Venezuela and Brazil is of interest and may be related to the Andean barrier between them. L. gomezi may of course form a species complex (Feliciangeli 1997) and this should be clarified by isoenzymes and DNA fingerprinting of different populations.

Comparison of the chorionic patterns of the present species and other Lutzomyia species (Ward & Ready 1975, Zimmerman et al. 1977, Endris et al. 1987, Feliciangeli et al. 1993) shows characters in most cases compatible with systematic position. Species of subgenus Psychodopygus have "volcano-like or mountain-like" patterns, like L. panamensis. L. ovallesi has similar chorionic sculpturing ("connected parallel ridges") to L. evansi ("polygonal") (Feliciangeli et al. 1993) and to other species belonging to the Verrucarum group. However, chorion sculpturing pattern of L. verrucarum from a Peruvian Andes valley combines "connected ridges" pattern with "reticular" pattern (Pérez & Ogusuku 1997). Slight contrasts of eggs morphology described in L. verrucarum from different localities could be associated with geographical isolation of sand fly populations in different Andean valleys (Pérez & Ogusuku 1997). The eggs of L. venezuelensis, the only species of Micro-pygomyia subgenus previously studied (Feliciangeli et al. 1993), are in the "elliptical" category, and those of L. absonodonta, belonging to the same subgenus, in the "reticular" category. These two categories are very similar by virtue of a common basal structure. Connected parallel ridges give L. migonei eggs a polygonal aspect similar to those described, using light microscopy, for the eggs of L. lenti and L. bahiensis (Species Group Migonei) (Feliciangeli et al. 1993).

The morphology of the posterior pole, described for the first time in Phlebotomus eggs (Fausto et al. 1992), has not yet been used much as an ootaxonomic character of sand fly eggs: most morphological descriptions of Lutzomyia eggs lack this structural detail. However, as reported for other species (Fausto et al. 1992, 1993, Pérez & Ogusuku 1997), the posterior poles of the present five species show species-specific morphology, which could be important for sand fly ootaxonomy.

However, more information, especially regarding the egg morphology of other genera, is needed to indicate phylogenetic relationships among the sand fly taxa.

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This work has been funded by CDCHT, Universidad de Carabobo (FCS-91-044) and Italian MURST-ex40%.

Copyright 2001 Fundacao Oswaldo Cruz Fiocruz

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