In the hybrid zone of the two mouse subspecies Mus musculus musculus and Mus musculus domesticus, mice with hybrid genotypes harbour, on the average, more helminth parasites (cestodes and nematodes) than mice of the two parental taxa. In order to determine the roles played by genetic parameters in this phenomenon, mice with recombined and parental genotypes were experimentally infected with the intestinal pinworm Aspiculuris tetraptera, a natural parasite of the house mouse. The results showed that the high susceptibility of the hybrid zone mice is genetically determined. In addition, this study shows the occurrence of variability among resistant parental populations.

The blue mussel, Mytilus edulis, and the Mediterranean mussel, Mytilus galloprovincialis, occur widely over much of northern Europe, and wherever they are sympatric they hybridize. The hybrid zone is large, ranging from western France to the north of Scotland, and is spatially complex, containing a mixture of pure, hybrid and introgressed individuals. Results from an Irish study in 1981, using partially diagnostic allozyme markers, indicated that mussels on the Irish Sea coast were solely M. edulis, but on Atlantic wave-exposed shores, and to a much lesser extent on wave-protected shores, mussels comprised an interbreeding mixture of M. edulis and M. galloprovincialis. In this study mussels were analysed from 20 locations on Irish coasts, using the Me15/16 nuclear DNA marker. The results showed a high frequency of M. galloprovincialis (0.378 ± 0.198) and hybrid (0.429 ± 0.175) genotypes, and correspondingly low frequencies of the M. edulis genotype (0.194 ± 0.107) at both exposed and sheltered locations on Atlantic coasts, indicating no apparent advantage for the M. edulis genotype at wave-protected sites. Mytilus galloprovincialis was virtually absent from the Irish Sea. Mussels in this area may be a self-recruiting population of M. edulis due to thermal front development at the northern and southern entrances to the Irish Sea in late spring, thereby preventing an influx of spring-spawned Mytilus larvae. The apparent change in the genetic composition of mussels on Atlantic coasts since the early 1980s could be related to climate change, or to aquaculture practice in Ireland whereby mussels from exposed shores are used to seed ropes in wave-protected bays and estuaries.

Female-biased sex ratios in Danaus chrysippus (Linnaeus) (family Nymphalidae, subfamily Danainae, tribe Danaini) populations are attributed to the action of an endosymbiotic ‘male-killer’ bacterium of the genus Spiroplasma. In stark contrast to the extensive geographic range of their host, the Spiroplasma appears to be restricted to East Africa, where four African D. chrysippus subspecies exist sympatrically and form a hybrid zone. In this study, specimens collected at three sample sites within the hybrid zone were screened for Spiroplasma infection. The findings demonstrate that, within the hybrid zone, the frequency of Spiroplasma infection varies both spatially and temporally. Host genotype at three biallelic wing pattern loci, representative of subspecific genetic divergence, is correlated to Spiroplasma infection. Linkage between the frequency of the recessive a allele and Spiroplasma is established, suggesting the presence of a linked allele controlling infection susceptibility. In addition, a negative correlation between D. chrysippus forewing length and Spiroplasma infection is identified, suggesting that infection has a deleterious effect on body size.

The variation in 10 morphometric characters and the degree of sexual dimorphism in size and shape was studied in pure and hybrid populations of smooth Triturus vulgaris and Montandon's T. montandoni newts. Multivariate analyses showed pronounced interspecific differences in both sexes. Females differed mainly in traits related to general size, whereas males differed mainly in body proportions. Triturus vulgaris males possessed a relatively longer trunk and tail. Sexual size dimorphism was much stronger in T. montandoni, the females being the larger sex. In hybrid zone populations a general correlation between morphometric traits and nuclear genotypes was found. This does not, however, fully explain the variation in size and shape. Whereas the majority of genetically identified hybrids fell into the range of morphologically pure species, many genetically pure individuals were classified as morphometric intermediates. The linear relationship between the morphometric and genetic variation suggested no strong differential selection on the molecular markers and loci responsible for morphometric differences between species. The differences in the level of sexual size and shape dimorphism, however, suggest that females of both species show dissimilar mate preferences, and thus hybrid males may experience reduced fitness resulting from lower mating success.

Breeding data for pied flycatchers Ficedula hypoleuca and collared flycatchers F. albicollis, from 47 localities in the Czech Republic and Slovakia, were analysed. We show that co-existence and hybridization were mainly restricted to a rather narrow, latitudinal cline. The distribution of the two flycatchers coincides with topography and habitat, collared flycatchers dominating in warmer habitats than pied flycatchers. Maintained co-existence within the same locality was the exception rather than the rule and most matings occurred in allopatry. In sympatry hybridization occurred at a frequency that was much lower than expected from random mating. Hybrids had low hatching success but some hybrids were apparently fertile. Shortage of conspecific mates may explain why individual birds engage in mixed species pairs. The proportion of pied flycatchers that was involved in mixed pairs was high and increased with decreasing relative frequencies of pied flycatchers in the population. The proportion of collared flycatchers involved in mixed pairs was low, reflecting the higher relative frequency of this species in most mixed populations. Comparisons suggest that fewer hybrids are breeding in the Central European, clinal hybrid zone than in the isolated hybrid zones of the Baltic Isles. Moreover, hybrid fertility was apparently higher in the island zones than in the clinal zone. We suggest that more extensive introgression in the isolated island populations has resulted in an increase in hybrid fitness by an accumulation of fertile individuals of mixed ancestry (i.e. F2, F3 . . . Fn-hybrids) acting as bridges for gene exchange. Differences in the dynamics of the two classes of hybrid zones, especially in pattern of gene flow, may explain these differences in frequency and fertility of hybrids.