Morphometric variations of laelapine mite (Acari: Mesostigmata) populations infesting small mammals (Mammalia) in Brazil

The goal of this study was to evaluate the morphometric variation of laelapine populations (Acari, Mesostigmata) associated with neotropical oryzomyine rodents at different geographic localities in Brazil. Three nominal mite species were selected for study, all infesting the pelage of small mammals at different localities in Rio de Janeiro, Espírito Santo, Bahia, and the Federal District, Brazil. To analyse morphometric characteristics, thirty-seven morphological characters distributed across the whole body of each specimen were measured. We use the Analysis of Principal Components, extracting the three first axes and projecting each mite in these axes. Major species level changes in the taxonomy of the host mammals allows an independent examination of morphometric variation of mites infesting a set of distinctly different host species at different geographic localities. Gigantolaelaps vitzthumi and Laelaps differens are associated with oryzomyine rodents of the genus Cerradomys, and consistently showed a tendency to cluster by host phylogeny. Laelaps manguinhosi associated with Nectomys rattus in central Brazil is morphometrically distinct from mites infesting N. squamipes in the coastal restingas of Rio de Janeiro and Espírito Santo. The results obtained here indicate that laelapine mite populations can vary among geographic areas and among phylogenetically related host species. Clearly, the study of these mites at the population level can be an important tool for clarifying the taxonomy of both mites and hosts.


Introduction
The evolutionary expression of individual attributes is variable through time in biological systems. Species change continually, modifying the morphology and physiology of the individuals of each population, in response to characteristics of both the physical environment and the interactions with other organisms (Ricklefs, 1993). Among ectoparasitic arthropods, both short life cycles and reduced time between generations allow strong alterations in the populations and fast differentiation among them, especially when subjected to different selective pressures (Price, 1976). One of the consequences of the isolation of subpopulations and their independent evolutionary change is speciation. The increasing specialisation of diverging host species may increase the level of specialisation of parasites infesting these host groups (Price, 1976;Ricklefs, 1993;Begon et al., 1996). Mathematical analyses have been widely used in entomology and acarology to detect morphometric variations among populations and consequently, have clarified the dynamics of their speciation (Albrecht, 1979;Foottit and Mackauer 1990;Querino and Zucchi 2002). Multivariate statistical techniques are frequently used to order large morphometric datasets. In this way, the biological parameters producing morphological variation among individuals or groups can be resolved (Albrecht, 1979).
Species of the genus Gigantolaelaps Fonseca, 1939 are nidicolus mites of wild rodents from Central and South America, characterised by their great size (dorsal shield length more than 1400 µm) (Furman, 1972). This genus shows a predilection for oryzomyine rodents (Cricetidae, Sigmodontinae), and tends to be specific to these hosts (Guitton et al., 1986;Gettinger, 1987;Linardi et al., 1991;Barros et al., 1993;Martins-Hatano et al., 2002;Bittencourt and Rocha 2003). Gettinger (1992a); Gettinger and Owen (2000) have demonstrated that morphometric variations can occur among laelapine mite populations associated with different host species. This research implies that the morphologic characteristics of these mites can be influenced by the species identity of the host and that morphometric data can provide an important tool to clarify the taxonomy of both mites and hosts.
When this study was carried out, we had strong collections of Gigantolaelaps vitzthumi Fonseca, 1939, Laelaps differens Fonseca, 1936, and L. manguinhosi Fonseca, 1936, from coastal localities of restinga in Rio de Janeiro, Espírito Santo, and Bahia states, from the senior author's (FMH) doctoral project, and then also, from the cerrados and gallery forests of central Brazil, from another doctoral project (DG), near Brasília. Two host "species" that we had in common were "Oryzomys" subflavus and Nectomys squamipes. The goal of the original study was to evaluate the morphometric variation of laelapine populations associated with a single host species in different geographic areas. At that time, our G. vitzthumi and L. differens collections from all these localities were considered hostspecific associates of a single widely distributed host species, Oryzomys subflavus; and Laelaps manguinhosi, a specific associate of Nectomys squamipes, another wide-ranging host. We were interested more in geographic variation in mite populations than genetic variation of the hosts. However, following major revisionary changes in the taxonomy of the rodent tribe Oryzomyini (Weksler et al., 2006), and the genera Cerradomys (Percequillo et al., 2008;Tavares et al., 2011)

Material and Methods
The mites and hosts were collected in four different geographic areas and at different time periods. The areas were: 1) the Restinga de Jurubatiba National Park, in Rio de Janeiro State (RJ), 2) Restinga of Praia de Neves, in Espírito Santo State (ES), 3) Distrito Federal (DF) and 4) the Chapada Diamantina, in the state of Bahia (BA).

Statistical treatment
Ordination techniques consist of projecting objects graphically along axes representing a relational ordinate, or forming a dispersion diagram with two or more axes (Legendre and Legendre, 1988). The graphical axes are chosen to represent the greatest fraction of variability of given multidimensional data, in a space with reduced dimensionality related to the set of original data (Legendre and Legendre, 1988).
To reduce dimensionality of the morphometric data we use the Analysis of the Principal Components, extracting the three first axes and projecting each mite on these axes (Legendre and Legendre, 1988). To accomplish this analysis, we used the Systat 11.0 program.

Laelaps differens Fonseca 1936 and Laelaps manguinhosi Fonseca 1936
The genus Laelaps Koch 1836 is a large and diverse group of mites of cosmopolitan distribution and generally associated with myomorph rodents, particularly the families Muridae and Cricetidae (Radovsky, 1985;Tipton, 1960). Laelaps manguinhosi frequently infests semiaquatic rodents, like Nectomys and Holochilus (Furman, 1972). Furman (1972) described two subspecies for L. manguinhosi, indicating that this author was aware that variations could occur in this species.
The genus Nectomys has a wide geographical distribution in South America, from Guianas to Paraguay and northeastern Argentina (Musser and Carleton, 1993;Eisenberg and Redford, 1999). Genetic and morphologic studies of the Brazilian populations of this rodent demonstrate that there are two nominal species: Nectomys squamipes and N. rattus . Twenty-one mites were measured (10 from the Restinga of Jurubatiba and 4 from Praia de Neves associated with N. squamipes, and 7 from Distrito Federal associated with N. rattus) ( Table 1). For Laelaps manguinhosi specimens collected from N. squamipes in RJ and ES clustered together, while the Distrito Federal specimens from N. rattus showed a strong tendency to be separate on the third axis (Figure 3).  Eigenvalues of the three first axes were 11.54; 5.53 and 3.6 and the total variation explained was 64.6% (36.1%, 27.0% and 11.2%) ( Table 2). Laelaps differens is a closely related species of L. manguinhosi and its occurrence on the species of the former "Cerradomys subflavus" group has been recorded in several geographic localities (Gettinger, 1992a). Thirty individuals of this mite species were measured, including 14 from Restinga de Jurubatiba, 10 from Praia de Neves, on C. goytaca and 6 from Distrito Federal, associated with C. scotti (Table 1). Laelaps differens specimens from different areas formed three distinct groups, with the restinga localities (Jurubatiba and Neves) forming closer groups ( Figure 4). However, the mites are distinctly grouped, implying that C. goytaca populations from restingas in Espírito Santo and Rio de Janeiro may be more isolated than is presently known. Eigenvalues of the first three axes were 11.29, 4.23 and 3.27, respectively. The total variation Table 2. Characters measured on the body of each laelapine mite of Laelaps differens, Gigantolaelaps vitzthumi and Laelaps manguinhosi from the four study areas and weight of each measure in the three first axes of the Main Components.

Discussion
The analysis of the morphometric characteristics of L. differens indicates that mite populations in the Distrito Federal are different from those in restingas of the Atlantic coast of Brazil. These variations may be related to environmental differences among areas, resulting in different selective pressures on populations. Both restinga populations also showed such separation, but on a smaller scale. The restinga areas, in consequence of the high degradation level of this environment, have been fragmented and isolated (Rocha et al., 2003). The lack of contact between host and, consequently, of the mites populations, may have led to a differentiation among these populations. The importance in geographic factors is poorly understood, and often difficult to evaluate because of the lack of knowledge of the species boundaries among the host mammals (Furman, 1972;Gettinger and Owen, 2000).
The population of Gigantolaelaps vitzthumi from Chapada Diamantina formed a distinct group. This population is associated with C. vivoi, and karyotypically distinct from specific populations associated with C. scotti from the Distrito Federal and C. goytaca from Restinga de Jurubatiba. This indicates that the mite physical characteristics are strongly dependent of the particular host species with which it is associated. This result illustrates the potential of this study as a tool in elucidating taxonomic relationships of the hosts. Lizaso (1968) described a new species of the genus Gigantolaelaps (G. bahiensis) associated with "O. subflavus" in Feira de Santana, BA. Only one individual mite of the new species was collected and, typical females of G. vitzthumi were found on the same host individual. Furman (1971Furman ( , 1972 pointed out that morphologic variations are commonly observed in the genus Gigantolaelaps, especially related to body size, and could be associated with differences in the age or food quantity; Furman placed Lizaso's species in synonymy with G. vitzthumi. Our studies indicate that further morphometric analysis and karyotypes of both, mammals and mites, may elucidate whether this mite represents a species distinct from G. vitzthumi and if the host is the recently described species Cerradomys vivoi. It is possible that when carefully examined and measured, Lizaso's G. bahiensis is a distinct species infesting the different host species, C. vivoi. The morphometric variations between populations of G. vitzthumi from Restinga de Jurubatiba and from Distrito Federal are not clear in contrast with that observed for L. differens. However, the characters used by Gettinger (1992a) as basic for the morphometric studies of laelapine mites are better applied with Laelaps the smaller species. Gigantolaelaps species have very long setae, which are broken easily during the processing for slide mounting. For this reason, a new set of morphological characters may be necessary with these large mites, to detect subtle variations in morphology among closely related hosts.
In Paraguay, Gettinger et al. (2011) found L. manguinhosi associated with five species of rodents in palustrine habitats, including Nectomys, Holochilus, Pseudoryzomys, and Sooretamys. Morphometric studies support the view that this mite was not a single monomorphic species capable of infesting a wide range of host species that share habitats, but is a complex of morphologically similar mite species, each infesting a distinct host species. Our studies in Brazil further verify these relationships, and show that morphometric variation can be measured between species of congeneric Nectomys.
The results obtained at present study indicate that laelapine mite populations can vary among geographic areas and, especially, among phylogenetically related host species. Clearly, the study of these mites at the population level is fundamental to help solving problems in evolutionary biology, especially related to the specificity relations with their hosts (Gettinger, 1992a). The morphometric analysis of laelapines associated with different species of Cerradomys can assist in the differentiation of both the host and mite species, and in the delimitation of their geographical distributions.