We analysed the polymorphism of cross-amplified microsatellite loci in two endangered butterflies of the genus Phengaris, which inhabit warm grasslands. Specimens of P. arion and P. 'rebeli' collected in Poland showed contrasting levels of variability in the investigated loci. All six tested microsatellites were highly variable in P. arion, whereas in P. 'rebeli' one locus was monomorphic and the other four loci showed a rather low level of polymorphism. Genetic differentiation was small or moderate among P. arion populations and high among P. 'rebeli' populations. We hypothesise that these differences reflect: (1) the relatively high genetic variability of P. arion compared with other members of the genus, indicated by previous studies on allozymes; (2) the small, in the majority of cases, genetic differentiation among populations of P. arion, which suggests recent gene flow; (3) a decrease in the genetic variability and increased isolation of peripheral populations of P. 'rebeli'. In both species, the occurrence of "null" alleles was suggested for some loci, which should be taken into consideration in future studies of the population structure. Additional analysis performed on the corrected frequency of alleles indicated that "null" alleles constitute only a minor problem when estimating genetic differentiation, using FST and AMOVA, however they strongly influence estimates of heterozygosity and FIS, and inferences about the number of genetic groups and assignment of individuals, especially in the case of 'P. rebeli'.
Larvae and pupae of the obligatorily myrmecophilous Phengaris (Maculinea) arion were found for the first time in nests of Myrmica rugulosa, M. hellenica and M. schencki at a site in eastern Poland. M. sabuleti, commonly considered as a major host of the butterfly and whose abundance is vital for the survival of P. arion populations, was shown to be very rare and not used. Estimated mean density of Myrmica nests was very low, which suggests that relatively vast areas are necessary to support a viable population of P. arion in similar sandy habitats. This may explain the decline and rarity of the butterfly in the whole region, even though its relationships with ants seem not to be very specific there.