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Population structure of Colorado Potato Beetle Leptinotarsa decemlineata Say in the South Urals

Maksim B. Udalov, Galina V.Benkovskaya
Institute of Biochemistry and Genetics,
USC RAS, 450054,
Russia, Bashkortostan, Ufa,
prospekt Octyabrya, 71

Tatiana L. Leontieva
Bashkortostan Agricultural University,
450001, Russia, Bashkortostan, Ufa,
50th - year of October street, 34

INTRODUCTION The Colorado potato beetle (CPB) Leptinotarsa decemlineata Say (Coleoptera, Chrysomelidae) is interesting and new object from the point of view of population studies. This species, being evolutionarily comparatively young, with the continuous processes of speciation (Ushatinskaya, 1981; Fasulati, 2000), is distinguishing by significant intraspecific polymorphism and, as a result, is characterized by striking ecological plasticity and adaptability. All this makes possible for it to enlarge successfully its area (Fasulati, 2002), being adapted to the new climatic conditions and the anthropogenic influence. In particular, in the territory of Russia this phytophagous insect already exceeded the boundaries of the assumed northern propagation on 200-300 km (Figure 1). Adaptation to the anthropogenic influence realizes as resistance to almost all existing at the present moment insecticides (The database of arthropods…). It becomes obvious, that for control and regulation of the number of this pest not enough only to develop new insecticides or to select the resistant strains of potatoes (including transgenic strains).

It is known that the population is the unit of control both of economically "useful" species and "pests" (Yablokov, 1987; Altukhov, 2003). Consequently, it is necessary to study CPB at the population level, in the first place, for the purpose of monitoring the processes of adaptations forming (in particular - the development of resistance to the insecticides), and in the second place, for search and optimization of the methods of microevolutionary processes analysis in the concrete places of CPB inhabiting - under conditions of the South Urals in our case.

For evaluating the degree of polymorphism and study of the microevolutionary processes in the populations of CPB it is possible to use the phenetic methods, based at the analysis of the morphs frequency of imago (cover spots of pronotum, elytra, head, the abdomen), nervation of wings, pigmentation and the phenes of the cover spots of larva, pigmentation of eggs. Is assumed that the given signs are hereditary caused, in particular - cover spots of pronotum (Gritsenko et al., 1998) and elytra (Ovtchinnikova et al., 1982). This approach makes it possible to differentiate clearly the structure of the CPB populations in its area. In particular, in the territory of Eurasia S.R. Fasulati demonstrated (analyzing the morphs frequency of pronotum) seven population complexes associated to the basic agroclimatic zones (Methodical recommendations…, 1993; Vilkova et al., 2005), which are biotaksonomically "... apparently, are close to the geographical races in K.M.Zavadsky's classification..." (Zavadskiy, 1967; Fasulati, 2002).

Thus, the purpose of this work was the analysis of the polymorphism of CPB in the South Urals (Republic Bashkortostan) with the applying of of phenetic analysis methods and the emerging of its formed at present population structure.

MATERIALS AND METHODS In the work were used the imago of CPB, collected in the territory Bashkortostan in 2002 - 2005 years. The places of collections in local populations were indicated in Table 1. For the phenetic analysis were used the phenes of vortex (Klimets, 1988; Benkovskaya et al., 2006), back of the head (Benkovskaya et al., 2006), pronotum (Fasulati, 1985) and elytra (Klimets, 1988; Benkovskaya et al., 2006), in sum 22 variations.

The analysis of the intra-population variety, based on the phenetical structure of local populations, was conducted with the applying of two indices, proposed by L.A. Zhivotovskiy - average number of variations µ and the portion of the rare variations h:
where m is quantity of variations in the population, p - frequency of variation (Zhivotovskiy, 1982).

Estimation of the interpopulation differences. The index of the similarity among populations r was used for calculating the genetic interrelations between the local populations of CPB: where m - quantity of variations in population; pm, qm - frequency of variations in the compared populations (Zhivotovskiy, 1982).

The dendrograms of phenetical interrelations between the local populations of CPB were constructed by the values of index r with using the STATISTICA for Windows 5.0 (1984-1985, Statsoft, Inc.) software package. The clustering was performed by Ward's method, as a distance measure were used Euclidean distances.

RESULTS AND DISCUSSION During the first stage of our investigation were analyzed the frequencies of the phenes of the cover spot patterns of four body counterparts - vortex, back of the head, pronotum and elytra. In this case, according to the results of phenetical analysis were calculated the matrices of the phenetic distances r between the samples from the local populations. Constructed according to these distances the dendrogram of phenetic interrelations is given in Fig. 2, to the left.

As a whole, according to the results of cluster analysis, in the Bashkortostan it is possible to isolate two large intra-population groups, which include: (a) local populations from the center of the examined territory and (b) local populations from the periphery of territory (Fig.2, to the right). These group of populations besides their geographical localization are differing by the level of intrapopulation variability. For the cluster (a) µ=3.628, h=0.347, for the cluster (b) µ=3.934, h=0.286.

It is possible that in the initial stage of the formation of the CPB population as basis for its structure differentiations served the first hotbeds of its appearance (Alsheevskiy, Al; Kumertauskiy, K, and Arkhangelskiy, Ar, regions), i.e., was exhibited the kind of the principle of founder. As the indirect confirmation of the influence of the principle of founder on further formation of population structure in South Urals can serve the fact that on the basis of the phenes frequencies comparison of cover spots of the samples number in our laboratory was already assumed about the settling of the larger part of the Bashkortostan territory by CPB from the southern (steppe) center of settling (Fig 2, to the right) (Benkovskaya et al., 2000).

The effect of founder proved to be so strong that it not was levelled by the subsequent settling of CPB from the West and, possibly, it was intensified by the selective action of insecticidal press (unpublished data).

At the same time, according to the evolutionary concept of Wright (1951), the subdividing of population to a number of the local populations contributes to the maintenance of considerably larger genetic variability in comparison with the united panmictic population. It is considered that this variety makes possible for the subdivided population to react more effectively to the environmental changes and, being adapted to them, to change its genotypic structure (Kokhmanyuk, 1982).

From the onset of a CPB to the territory Bashkortostan it passed about 30 years. During this time in its population was formed the subdividing (on the basis of the adaptations of gene pool to the determined environmental conditions), that contributing to more effective reaction to environmental changes, in particular - to the anthropogenic influences in the form of insecticides applying.

Possibly, this was one of the mechanisms, which brought to the kind of "half-isolation" (in Wright - Altukhov terms), which is been one of the fundamental characteristics of the subdivided natural populations (Altukhov, 2003). Thus, data of the analysis of the phenetical structure of the examined samples of CPB allow to isolate in the Bashkortostan territory at least two forming intrapopulation groups. These groups we have named conditionally "central" (a) and "peripheric" (b). The presence of two groups of CPB determines the necessarity of the differentiation for plant protection schemes. The basis of this differentiation must be the requisite yearly monitoring of resistance and dynamics of phenetic composition in the populations of Colorado potato beetle.

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