Marina B. Chiappero

Global genetic diversity of Aedes aegypti

Mosquitoes, especially Aedes aegypti, are becoming important models for studying invasion biology. We characterized genetic variation at 12 microsatellite loci in 79 populations of Ae. aegypti from 30 countries in six continents, and used them to infer historical and modern patterns of invasion. Our results support the two subspecies Ae. aegypti formosus and Ae. aegypti aegypti as genetically distinct units. Ae. aegypti aegypti populations outside Africa are derived from ancestral African populations and are monophyletic. The two subspecies co‐occur in both East Africa (Kenya) and West Africa (Senegal). In rural/forest settings (Rabai District of Kenya), the two subspecies remain genetically distinct, whereas in urban settings, they introgress freely. Populations outside Africa are highly genetically structured likely due to a combination of recent founder effects, discrete discontinuous habitats and low migration rates. Ancestral populations in sub‐Saharan Africa are less genetically structured, as are the populations in Asia. Introduction of Ae. aegypti to the New World coinciding with trans‐Atlantic shipping in the 16th to 18th centuries was followed by its introduction to Asia in the late 19th century from the New World or from now extinct populations in the Mediterranean Basin. Aedes mascarensis is a genetically distinct sister species to Ae. aegypti s.l. This study provides a reference database of genetic diversity that can be used to determine the likely origin of new introductions that occur regularly for this invasive species. The genetic uniqueness of many populations and regions has important implications for attempts to control Ae. aegypti, especially for the methods using genetic modification of populations.

Genetic structure of Aedes aegypti in the city of Córdoba (Argentina), a recently reinfested area.

To understand the transmission of a vector-borne disease, knowledge of the magnitude of dispersal among vector populations is essential because of its influence on pathogen transfer. The principal vector of dengue, the most common arboviral disease in the world, is the mosquito Aedes aegypti (L.). This tropical and subtropical species is native to Africa but has dispersed worldwide since the XV century. In Argentina, the species was declared eradicated in 1963, but has reinfested the country in recent years. In the present work, we used RAPD-PCR markers to assess the levels of genetic variability and differentiation among populations of Ae. aegypti (the vector of dengue and yellow fever) in Córdoba, the second largest city in Argentina. We detected similar levels of genetic variability (He between 0.351-0.404) across samples and significant genetic differentiation between most population pairs within the city (F ST between 0.0013-0.0253). Genetic distances indicate that there are three distinct groups, formed predominantly by populations that are connected by, or near, main roads. This suggests that, in addition to other factors such as availability of oviposition sites or step-by-step migration, passive transport plays an important role in gene flow within the city.

Corn mice (Calomys musculinus) movement in linear habitats of agricultural ecosystems

Abstract Movement patterns of Calomys musculinus in linear habitats were studied in relation to sex, season, year, abundance, and width of border. Movement distances (MDs) were measured by seasonal capture, mark, and recapture samples during 2 years in wide and narrow borders in agroecosystems. The smallest MDs were registered in autumn and the highest in spring. In the breeding period MDs of males were larger than those of females. In the nonbreeding period MDs were similar between sexes. Effects of sex on MD were consistent with the promiscuous mating system of C. musculinus. MDs were greater in narrow than in wide borders. The narrowness of suitable habitats would force the mice to extend foraging trips in narrow borders. Opportunistic behavior of C. musculinus allows it to use linear habitats in a similar way to the 2-dimensional natural habitats.

Gene Flow Among Calomys Musculinus (Rodentia, Muridae) Populations in Argentina

Calomys musculinus is a Sigmodontinae rodent inhabiting periodically disturbed habitats in the central eastern plains of Argentina. It is the natural reservoir host of Junin virus, the etiological agent of Argentine Hemorrhagic Fever (AHF). In order to analyze the levels of gene flow among populations of this species, allozymic variability at 26 loci was studied in 291 individuals from the endemic zone of AHF and localities outside it. All populations showed high levels of polymorphism (He between 0.107 and 0.144; P95% between 38 and 54%). Individual loci f values were in most cases negative, although not significantly different from zero. Mean genetic differentiation among populations was low, but statistically significant (Θ = 0.020; P < 0.01). There was no correlation between genetic and geographic distances between pairs of populations, and scatter of the pairwise points suggests that, at the regional scale, genetic drift is more influential than gene flow. This result can be interpreted as indicative of a relatively recent expansion of C. musculinus habitat and restricted on-going gene flow, which would be compatible with a relatively slow expansion rate of AHF.

High genetic differentiation among populations of the small cavy Microcavia australis occupying different habitats

The small cavy Microcavia australis, a social and fossorial rodent, inhabits a large distribution range in South American arid zones. The species is versatile in coping with the seasonal and spatial variability typical of these environments through changes in morphology, physiology, and behavior. In order to explore whether phenotypic variations are related to the evolutionary history of the species, we analyzed the levels of genetic variability and divergence among four populations that differ in climate and habitat characteristics, two belonging to highlands and the other two from lowlands. We sequenced the mitochondrial control region and used the Inter Simple Sequence Repeats technique to study variability in the noncoding nuclear genome. Results from both genetic markers were consistent. Variability levels were high for all populations, and even higher for lowland ones. Pairwise genetic differentiation varied greatly, all comparisons being statistically significant except for the two highland populations. Seventeen haplotypes were detected which displayed three clear lineages: two corresponding to each lowland population and one to those in the highlands. Levels of genetic differentiation between population pairs varied widely. Haplotypes showed a mean sequence divergence of 1.4% between lowland populations and 0.2% between highland ones, whereas divergence was around 9% when populations from different altitudes were compared. Results from BEAST analysis support extant hypotheses suggesting that lowland forms are clearly older than the highland group. The deep genetic divergence between lineages poses the need to search for new evidence for properly defining the taxonomic status of divergent populations of M. australis.

TEMPORAL FLUCTUATION OF EFFECTIVE SIZE IN POPULATIONS OF CALOMYS MUSCULINUS (MURIDAE: SIGMODONTINAE)

Abstract Calomys musculinus is the natural reservoir of Junin virus, the etiological agent of Argentine hemorrhagic fever. In this paper we measure the effective size of 2 populations of the rodent over a 2-year period. Twenty enzyme-coding loci were analyzed using vertical starch gel electrophoresis. Effective population sizes (Nes) were estimated by the pseudolikelihood method in 2 populations 280 km apart in central Argentina. Both populations experienced marked seasonal changes in relative density and in Ne (between 19.8 and infinity). Changes in percentage of polymorphic loci and mean number of alleles per locus were statistically significant and were roughly correlated with density and Ne. Observed changes in heterozygosity, in contrast, were not significant. After low-density periods, mixing of surviving individuals coming from different demes may play an important role in the maintenance of variability and recovery of Ne in populations of C. musculinus.

Genetic structure of populations of Calomys laucha (muridae, sigmodontinae) from central Argentina

Abstract The genetic structure of populations of the Sigmodontinae rodent Calomys laucha was studied by means of allozyme electrophoresis. This highly opportunistic species is found preferably in periodically perturbed habitats of crop fields in central Argentina, where it can attain very high densities. A total of 17 enzymatic proteins assayed gave information on 25 loci; only four were monomorphic in the seven populations studied. Levels of genetic variability (H o from 0.144 to 0.171; P 95% from 44% to 56%) were higher than mean values reported for mammals and rodents. These high levels of heterozygosity could be maintained by large populations that do not experience great fluctuations in size, or by a social structure consistent in many small breeding groups that are formed and dissappear every breeding season. Genetic differentiation at a macrogeographical scale (θ=0.018) was low but statistically significant, and showed no correlation with geographic distance between pairs of populations. The pattern of population differentiation found is compatible with a relatively recent range expansion.

Multiple Paternity in a Wild Population of the Corn Mouse: Its Potential Adaptive Significance for Females

Mating with multiple males within a single reproductive event is a common female mating strategy in mice and voles, but its adaptive function is often unclear. In this study, we used 7 microsatellite loci to investigate if multiple paternity occurs in wild populations of the socially promiscuous Calomys musculinus. We also analyzed if multiple paternity increases litter size and/or genetic variability among offspring. In addition, we examined if multiple paternity occurs more frequently in litters conceived at high population density than those conceived at low population density. By genotyping 23 females and their 135 embryos (5.9 ± 1.6 SE pups per female), we found that 56.5% of the litters were sired by 2 or 3 males. We found no association between multiple paternity, litter size, and genetic variability. In addition, multiple paternity did not vary in relation to population density. Our results provide clear evidence of multiple paternity and offer the first genetic documentation of mating systems in mice in Argentina. We discuss the extent of multiple paternity in relation to potential adaptive strategies in female corn mouse. El apareamiento con múltiples machos durante un solo evento reproductivo es una estrategia común de acoplamiento en hembras de ratones y ratas de campo, sin embargo su función adaptativa a menudo es poco clara. En este estudio, hemos utilizado 7 loci de microsatélites para investigar si existe paternidad múltiple en poblaciones silvestres del ratón maicero (Calomys musculinus) especie socialmente promiscua. También analizamos si la paternidad múltiple aumentaó el tamaño de la camada y/o su variabilidad genética. Además, se analizó si la paternidad múltiple ocurre con más frecuencia en camadas concebidas a alta densidad poblacional, que las concebidas a baja densidad poblacional. El genotipado de 23 hembras y sus 135 embriones (5.9 ± 1.6 ES crías por hembra) demostró que, al menos el 56.5% de las camadas son engendradas por dos o tres machos. No se encontró asociación entre la paternidad múltiple, tamaño de la camada y la variabilidad genética. Además, la paternidad múltiple no varió en relación con la densidad de la población. Nuestros resultados proveen clara evidencia de paternidad múltiple y ofrecen la primera documentación genética de sistemas de apareamiento en ratones en Argentina. Discutimos el alcance de la paternidad múltiple con relación a potenciales estrategias adaptativas en hembras del ratón maicero.

Allozymic polymorphism and genetic differentiation among populations of Calomys musculinus and Calomys laucha (Rodentia: Muridae) from eastern Argentina

Summary The genetic variability and divergence among natural populations of Calomys musculinus and C. laucha from eastern Argentina were examined by protein electrophoresis of 24 loci. High levels of genetic variability were found in both species when compared to other rodents and mammals. Mean expected heterozygosity (He) ranged from 0.112 to 0.156, proportion of polymorphic loci (P 95% ) from 37.5% to 50% and mean number of alleles per locus (A) of 1.6-1.7 for C. musculinus . H e from 0.097 to 0.126, P 95% from 34.8 to 39.1 and A from 1.7 to 1.9 were the values for C. laucha . Populations of this last species revealed a higher degree of geographical differentiation (Θ = 0.017, P C. musculinus (Θ = 0.002, P > 0.05). These results are in agreement with the known social structure and habitat of both species. C. musculinus is an opportunistic species, with a loose social structure and a predominance of transient over resident animals, indicating a high ambulatory activity. C. laucha , on the contrary, inhabits almost exclusively cultivated fields, and evidence of social stratification has been reported for this species.

Genetic Evidence of Expansion by Passive Transport of Aedes (Stegomyia) aegypti in Eastern Argentina

Fil: Diaz Nieto, Leonardo Martin. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones En Biodiversidad y Biotecnologia; Argentina. Fundacion para Investigaciones Biologicas Aplicadas; Argentina