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Genetics and Molecular Biology

Print version ISSN 1415-4757On-line version ISSN 1678-4685

Genet. Mol. Biol. vol. 21 n. 2 São Paulo June 1998

http://dx.doi.org/10.1590/S1415-47571998000200002 

Chromosomal variation, macroevolution and possible parapatric speciation in Mepraia spinolai (Porter) (Hemiptera: Reduviidae)

 

Daniel Frias and Juan Atria
Instituto de Entomologia, Universidad Metropolitana de Ciencias de La Educacion, Av. José Pedro Alessandri, 774, Casilla 147, Santiago, Chile. Fax: 56-2-2749043,
E-Mail: danfrias@discovery.umce.cl   Send correspondence to D.F.

 

 

ABSTRACT

Mepraia spinolai is an endemic species in Chile that lives in wild and domestic habitats. It is the only species of the Reduviidae family that shows alate polymorphism; females are always wingless, but males can be found with and without wings. The M. spinolai karyotype consists of 10 pairs of autosomes and a complex sex determination system. Males from the northernmost regions I and II (latitude 18°-26° South) are always winged (braquipterous) and are X1X2Y, with a large Y chromosome. From region III to the metropolitan region (latitude 26°-33° South), males may be either winged or wingless but appear to be polymorphic for a small neo-Y chromosome, which may have originated by fracture of the large holocentric Y chromosome found in populations from farther north. Experimental crosses suggest that the genes for wings are linked in the Y chromosome and also that there are two cytologically indistinguishable types of neo-Y chromosomes. One form (Y1) bears a gene or genes for wings while the other (Y2) lacks such genes. Males that are X1X2Y1, X1X2Y1Y1 and X1X2Y1Y2 are winged, while the absence of Y1 (X1X2Y2 and X1X2Y2Y2 ) results in a wingless male. These chromosomes and morphological changes are correlated with a shift of the southern population into more arid habitats of the interior in the metropolitan region and region III.

 

 

INTRODUCTION

The main domestic vector of Chagas' disease in Chile is Triatoma infestans, but the endemic species, Mepraia spinolai, found in wild and domestic habitats, can be a vector of this malady (Schofield et al., 1982, Frías and Kattan, 1989).

M. spinolai was originally described as Triatoma spinolai (Porter, 1933). The presence of an alate polymorphism resulted in taxonomic confusion, and Mazza et al. (1940) placed this species in the monotypic genus Mepraia. However, his name was not recognized (Frias et al., l987). Recently, Lent et al. (1994) reexamined the genus Triatoma. They recognized the monotypic genus Mepraia as distinct from Triatoma because of a combination of many morphological and male genital differences.

The evolution process is often accompanied by structural rearrangement of the genome (White, 1973, 1978). The nature of these rearrangements is determined by chromosome architecture and behavior. Certain patterns which commonly occur in some taxa are rare or never occur in others. Thus, in organisms with localized centromeres, acentric fragments are lost during cell divisions. The process of fragmentation contributes to evolutionary changes in the karyotype only if such fragments fuse with a competent chromosome or fragments with a centromere. However, this limitation does not apply to members of the genus Mepraia. In these organisms chromosomes have diffuse kinetochores. Chromosome fragments are able to form spindle attachments and migrate autonomously to the poles during cell divisions. In lineages with holokinetic systems, the production of neo-chromosomes may originate as fragments resulting in a process of karyotypic evolution not possible in other taxa (White 1973; Thomas, 1987; Panzera et al., 1992, 1995).

Chromosome studies in the Reduviidae show that 2n = 26 is the most common chromosome number. The sex determination system is primarily XY, although more complex systems such as X1X2Y, X1X2X3Y, and others are commonly encountered (White, 1973). Although previous cytogenetic studies of Triatoma species show that the common diploid chromosome number is 2n = 22 with an XY sex determination system, other more complex sexual chromosome mechanisms are encountered in this genus (Barth 1956a,b; White, 1973; Maudlin, 1974; De Vaio et al., 1985; Thomas, 1987; Panzera et al. 1992, 1995; Usinger et al., l966). Panzera et al. (1996) reported that the M. spinolai karyotype has 20 autosomes and an X1X2Y sex determination mechanism.

Recently, we have discovered other types of sex determination mechanisms which appear to be associated with modification of wing morphology. Here we present the results of a recent study of the karyotype variation among M. spinolai populations, which establishes the geographic relationship between divergence in wing morphology and sex determination mechanisms.

 

MATERIAL AND METHODS

Populations studied

Specimens used in this study were collected from populations in natural and domestic habitats in the following localities: Region I: Caleta Vitor, located on the coast on a marine bird nest 65 km south of Arica city (4 male and 3 female specimens); Punta de Lobos, located 1 km south of Rio Seco (5 males, 4 females). Region II: Piedras Negras, associated with lizards located near the coast 60 km north of Tocopilla city (5 males and 5 females); Caleta Bandurrias, 21 km south of Paposo (3 males and 3 females). Region III: Parque National Pan de Azucar, Cerro el Soldado (1 male); Isla Pan de Azucar (2 males and 2 females); Inca de Oro, Mina San Vicente (3 males and 2 females), 16 km northeast of Estación Chimbero. Region IV: Pueblo Hundido, 3 km south of Combarbala city, domestic site associated with goats (5 males and 2 females); Mina Monte Verde, 12 km west of Illapel, copper mine and goats (4 males and 1 female). Region V: El Melón,12 km northeast of Nogales. Metropolitan region: TilTil, 55 km northwest of Santiago in wild rodent nest of Octodon degu (5 males and 3 females); Colina, Canteras de Quilapilun, 41 km north of Santiago City (5 males and 3 females).

Cytological preparation

M. spinolai karyotypes were obtained from adult gonads stained and fixed by the classic lacto-acetic orcein squash method (Frias et al., 1982). The barium hydroxide method from Sumner (1972) was used for C banding. To study the relationship between karyotype variation and alate condition, we analyzed the chromosomes of wingless and winged males. Meiosis was examined in special profase I, metaphase I and II and anaphase I and II. Some of the C banding preparations were obtained with a laser scanning microscope 633 (Helium-Neon, Carl Zeiss, Inc., Model 210 from the Michigan State University) and others with optical Leitz Laborlux K microscope from Instituto of Entomología, UMCE.

Cross-breeding experiments

To understand the heritability of the alate condition in males, we crossbred females with wingless and winged males and established wing frequency in their F1 progeny. Moreover, the parent and F1 male karyotypes were analyzed. The following crosses were made according to type and collection site: A: 21 wingless males x wingless females (Colina, 7; Til-Til, 11; Combarbala, 2, and Mina Monte Verde, 1). B: 8 winged males x wingless females (Combarbala, 5; Mina Monte Verde, 2 and Til-Til, 1).

 

RESULTS

Geographic distribution

Figure 1 shows the pattern of geographic variation in male morphology. The northernmost populations are restricted to a very narrow zone along the coast between 18° latitude south and 26° latitude south. Below 26° latitude, populations extend along the coast without geographic barriers and expand to east into the dry valleys of the Andes mountains.

 

Figure 1 - Map of the geographic distribution of male wing morphology variation in Mepraia spinolai.

 

M. spinolai of the northern coastal zone (NCZ) in regions I and II

All males collected in the northern coastal zone were winged (braquipterous), while all females were wingless. Wingless males were never encountered in this region (Figure 1). Our cytogenetic results show that the karyotypes of populations in the NCZ (Caleta Vitor, Punta de Lobos, Piedras Negras and Caleta Bandurrias) consisted of ten autosome pairs and an X1,X2Y chromosome sexual system. The Y chromosome was C positive and larger than the X1 and X2 chromosomes (Figures 2 to 8). In these populations, analysis of sex chromosome segregations in anaphase II indicated that the large, C positive chromosome only segregated to one pole while X1 and X2 segregated jointly to the opposite pole (Figures 7 and 8). This confirms that the large, C staining body is the Y sex chromosome. In the majority of prophase, diplotene and diakinetic divisions, we observed only 3 sex chromosomes (Figure 9). In only one cell we observed four sex chromosomes of the same size (Figure 10). The extra chromosome probably represents a break of the holocentric Y chromosome (Figure 21). In leptotene of prophase I, the sex chromosomes were strongly heteropicnotic, and the autosomes appeared independently dispersed with very few telomeric or intercalary heterochromatic regions (Figure 11).

 

Figures 2-7 - Male meiosis of NCZ in regions I and II. C-Banding shows the big C-positive Y chromosome. 2: Metaphase I, Caleta Vitor population. 3: Metaphase I, Punta de Lobos population. 4: Metaphase I, Piedras Negras population. 5: Metaphase II, Piedras Negras population. 6: Metaphase II, Bandurrias population. 7: Anaphase II in lateral view, Bandurrias population.

 

 

Figures 8-13 - Male meiosis of NCZ and AZ populations. 8: C-Banding of anaphase II in polar view. It shows the segregation of a C-positive Y chromosome in a single pole and X1,X2 in the opposite pole in the Bandurrias population. 9: C-Banding prophase I, Diplotene shows the positive heteropicnosis of sexual chromosomes in the Caleta Vitor population. 10-13: Classic lacto-acetic orcein squash method. 10: Diplotene of prophase I shows 4 sexual chromosomes of similar size, X1,X2 and Y1 and Y2. The extra Y chromosome probably represents a break of the big Y chromosome. 11: Leptotene of prophase I of Caleta Vitor population shows the positive heteropicnosis of sexual chromosomes and absence of a chromocenter. The autosomes are scattered with very few intercalar or telomeric heterochromatic regions. 12: Metaphase I of micropterous male of Combarbala population shows X1,X2 and probably Y2 without a wing gene. 13: Metaphase II in macropterous male of Til-Til population probably with Y1 chromosome with wing gene. (a) Lateral view, (b) polar view of the same individual. In metaphase the X1,X2 and Y1 chromosomes are similar in size.

 

M. spinolai of the arid zone (AZ) (regions III, IV, V and metropolitan)

M. spinolai sex chromosomes in the arid region were polymorphic. Some winged and wingless males had 10 autosomes of similar size and an X1 X2 Y sex determination system. The size of the Y and X1 X2 chromosomes was similar, but the Y chromosome, which was C positive, segregated only to one anaphase II pole (Figures 12, 13, 14 and 15).

In 20 crosses of wingless males (with only one Y chromosome) x wingless females, all F1 male progenies were wingless (194 males and 200 females) and had only one Y chromosome (71 male karyotypes were analyzed). In a single cross between a winged male (with only one Y chromosome) x wingless females, all F1 male progenies were winged and had only one Y chromosome, similar to Figure 13.

Other winged and wingless males had an extra C positive Y chromosome (Figures 16 and 17). In anaphase II, the Y sex chromosomes segregated erratically (meiotic drift). Thus, in some cells there was only one Y chromosome and consequently in the other 3 sex chromosomes (X1X2Y) were present (Figure 18). However, other cells in the same individual may have 2 sex chromosomes. Some probably are X1X2 and others YY (Figure 19).

 

Figures 14-19 - Figures 14, 16, 18 and 19 were obtained with classic lacto-acetic orcein squash method. Figures 15 and 17 were obtained with C-banding method. 14: Anaphase II in polar view shows the segregation of sexual chromosomes in wingless male of Combarbala population. 15: Anaphase II in polar view shows the segregation of sexual chromosomes and probably C-positive Y2 chromosome in a wingless male of Til-Til population. 16: Metaphase II in a winged male of Combarbala population shows 4 sexual chromosomes (arrows) of similar size, X1,X2 and probably Y1 and Y2 or Y1 and Y1. 17: Gonial metaphase in a wingless male of Mina Monteverde population shows 2 C-positive Y chromosomes, small, similar in size, probably Y2,Y2. Moreover, the X1 presents a C-positive region. The autosomes in this population are C-positive in telomeric regions and are present in all AZ populations. 18: Anaphase II in polar view in the same individual as Figure 16 shows two sexual chromosomes (arrows), probably Y1,Y2 or Y1,Y1 or X1,X2. 19: Anaphase II in polar view in another cell of the same individual as Figure 16 shows the segregation of sexual chromosomes in one cell where there is only one Y chromosome. The other cell has X1,X2 and probably one extra Y chromosome Y1 or Y2 with erratic segregation in the meiosis.

 

In F1 progeny resulting from 7 crosses between winged males (with two Y chromosomes) x wingless females, from a total of 178 individuals, 65 winged, 30 wingless males and 83 females were observed. All the F1 wingless males had only one Y chromosome. Nevertheless, F1 winged males had one or two Y chromosomes in similar proportions.

In one single cross between wingless male (with two Y chromosomes ) x wingless female, all the F1 males were wingless with one or two Y chromosomes in similar proportion.

As in other Triatomine species, in AZ populations the sex chromosomes in leptotene of prophase I formed a chromocenter (Figure 20) and the autosomes of individuals in AZ had heterochromatin in telomeric region (Figures 17 and 20).

 

Figure 20 - Leptotene of prophase I shows a chromocenter formed with sexual chromosomes (a); telomeric C-positive regions (b) and association between autosomes by telomeric C-positive regions (c).

 

The origin of the extra Y sex chromosome appears to be from a break of the original large holocentric Y chromosome in NCZ populations (Figure 21). Thus, one form (Y1) bears a gene or genes for wings while the other (Y2) lacks such genes.

 

 

Figure 21 - The large holocentric Y chromosome of NCZ with the wing (w) gene locus, which after breaking the Y chromosome forms the neo-Y chromosomes, Y1 with the w gene and Y2 without the locus with the w gene. This probably caused the polymorphism for Y chromosome number in the AZ population.

 

DISCUSSION

The karyotype of individuals in the NCZ differed from the AZ population in several ways. In the NCZ the Y chromosome was large. There were X1X2Y sex chromosomes, and all males were winged. In the AZ, males may be winged or wingless, but appeared to be polymorphic for a small neo-Y chromosome which may have originated by fracture of the large holocentric Y chromosome found in populations from farther north. Experimental crosses suggested that there are two cytologically indistinguishable types of neo-Y chromosomes. One form (Y1) bears a gene or genes for wings while the other (Y2) lacks such genes. Males that were X1X2Y1, X1X2Y1Y1 and X1X2Y1Y2 were winged, while the absence of Y1 in X1X2Y2 and X1X2Y22Y2 individuals resulted in a wingless male. These chromosomal and morphological changes were correlated with a shift of the southern population into more arid habitats of the interior in the metropolitan region and regions III, IV and V. Moreover, there were striking differences in the color pattern of adults from the AZ and NCZ. This result suggests that the wingless condition has a genetic basis which is segregating either as a single gene with major effect or a group of linked genes on the neo-Y1 chromosome.

Crosses between NCZ individuals and those from the AZ indicate strong sexual isolation. When NCZ males were crossed with AZ females only a few F1 progenies were produced. The reciprocal cross (AZ males x NCZ females) also produced very few progenies. These observations strongly support the view that the two taxa are different species (Frias, D., Gonzáles, C. and Henry, A., unpublished results). The fragmentation of big holocentric C-positive Y chromosome of NCZ and the formation of neo-Y1 and neo-Y2 chromosomes detected in AZ was probably a very important chromosomal change in the evolutionary process and speciation of these NCZ and AZ populations.

Since there are no geographic barriers in the coastal zone, the speciation probably occurred in semigeographic condition in the framework of parapatric speciation. Moreover, the neo-Y2 chromosome without the wing gene originated wingless males that were apparently adaptable to natural AZ populations. This is consistent with thermoregulation hypothesis of the wings in insects. This hypothesis proposes that insect wings in hot places are shorter than those of individuals that live in cold habitats (Douglas, 1981; Kingsolver and Koehl, 1994).

 

ACKNOWLEDGMENTS

Helpful comments and reviews were provided by Guy Bush. We thank Vanderlei Martins and Gilberto Calvo for their comments, and Joanne H. Whallon for assistance in use of laser scanning confocal microscope at Michigan State University. Research supported by FONDECYT (No. 1940753-1994).

 

 

RESUMO

Mepraia spinolai é uma espécie endêmica no Chile que mora em habitats domésticos e silvestres. É a única espécie da família Reduviidae que apresenta polimorfismo de asas: as fêmeas são sempre sem asas, mas os machos podem ou não tê-las. O cariótipo de M. spinolai consiste de 10 pares de autossomos e um sistema complexo de determinação do sexo. Os machos das regiões mais setentrionais, regiões I e II (latitude 18o-26o S), sempre têm asas (braquíteros) e são X1X2Y, com um grande cromossomo Y. Da região III à região metropolitana (latitude 26o-33o S), os machos podem ou não ter asas, mas parecem ser polimórficos para um pequeno neo-cromossomo Y que pode ter se originado por fratura do grande cromossomo Y holocêntrico encontrado nas populações mais ao norte. Cruzamentos experimentais sugerem que os genes para asas estão ligados no cromossomo Y e também que há dois tipos citologicamente indistinguíveis de neo-cromossomo Y: uma forma (Y1) é portadora de um gene ou de genes para asas, enquanto que a outra (Y2) não apresenta tais genes. Os machos que são X1X2Y1, X1X2Y1Y1 e X1X2Y1Y2 têm asas, enquanto que a ausência de Y1 (X1X2Y2 e X1X2Y2Y2 ) resulta em um macho sem asa. Esses cromossomos e as modificações morfológicas correlacionam-se com uma mudança da população do sul para habitats mais áridos do interior da região metropolitana e da região III.

 

 

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(Received June 12, 1997)

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