Much interest has centred recently on the role of adaptive trade-offs between the immune system and other components of life history in determining resistance and parasite intensities among hosts. Steroid hormones, particularly glucocorticoids and sex steroids, provide a plausible mechanism for mediating such trade-offs. A basic assumption behind the hypothesis, however, is that steroid activity will generally correlate with reduced resistance and thus greater parasite intensities. Here, we present some findings from a field study of bank voles (Clethrionomys glareolus) in which we have looked at associations between parasite intensities, anatomical and morphometric measures relating to endocrine function and life history variation in three local populations inhabiting similar but mutually isolated woodland habitats. In general, sites with greater parasite intensities were those in which male C. glareolus had significantly larger adrenal glands, testes and seminal vesicles for their age and body size. Females also showed a site difference in adrenal gland weight. Some aspects of site-related parasite intensity were associated with asymmetry in adrenal gland weight and hind foot length, which may have reflected developmental effects on glucocorticoid activity.

Although the rarity of an endangered plant species can seldom be ascribed to its breeding system, knowledge of its breeding system may be critical to its recovery. The federally listed endangered Ziziphus celata (Florida ziziphus), a woody clonal shrub narrowly endemic to xeric upland habitats of the Lake Wales Ridge in central Florida, USA, is known from only five populations, four of which are sterile and perhaps uniclonal. Altogether only 11 genotypes of Florida ziziphus have been identified by allozyme electrophoresis. We studied the breeding system of Florida ziziphus and found that it is an obligate outcrosser and that some genotypes are cross-incompatible. We have been able to demonstrate the cross-compatibility of only 11 of the 44 test crosses performed to date (25%), as measured by fruit yield. Cross-incompatibility is most likely due to shared self-incompatibility (S) alleles among the few remaining genotypes of these fragmented populations. The identification and translocation of compatible mating types to create reproductively viable populations is essential for the recovery of Florida ziziphus.

We tested the null hypothesis that populations of hosts trapped in isolated neighbouring locations showing comparable habitat quality, should support similar helminth parasite communities. The study was undertaken in a 2-week period in late summer in NE Poland in a single year, thereby eliminating seasonal and between-year variation in parasite burdens. A total of 139 Clethrionomys glareolus (bank vole) were sampled from 3 forest sites of similar habitat quality. Total species richness was 11 (6 nematodes and 5 cestodes) with 85·6% of the voles carrying at least 1 species and an overall mean species richness of 1·4. At the component community level, the fewest species of helminths were recorded from site 2 ( n = 6, compared with 9 at each of the other sites), but site 3 had the lowest Berger-Parker Dominance Index and the highest Simpson's Index of Diversity. At the infracommunity level, site 3 had the highest mean no. of helminth species/vole, the highest mean Brillouin's Index of Diversity but the lowest mean no. of helminths/vole. Voles from sites 1 and 3 differed in the nematodes that were most common (site 1, Heligmosomum mixtum – 95%; site 3, Heligmosomoides glareoli – 79·3%). At site 2 no species exceeded 50% but prevalence of Syphacia petrusewiczi was higher than at the other sites. The prevalence of cestodes was too low to test reliably (12·9%), but the highest prevalence of adult cestodes was recorded at site 1 (22·5% compared with 4·9 and 1·7% for sites 2 and 3 respectively). Host sex did not influence infection, but mean species richness increased with age. The different sites were responsible for most of the variation in our data, and the intrinsic factors (sex and age) were less important in shaping the component community structure of helminths. We conclude that even locations in relative close proximity to one another (13–25 km), selected on the basis of similar habitat quality, have rodent populations that differ in their helminth parasite communities, although for reasons other than the factors quantified in the present study.

Ziziphus celata (Rhamnaceae) is an endangered shrub known from five sites in the Lake Wales Ridge region of Florida. Genetically effective population sizes are difficult to determine because Z. celata spreads clonally via root sprouts. Allozymes were used to investigate clonal structure and to describe genetic diversity within the species. Five (25%) of the 20 putative loci resolved for 13 enzyme systems were polymorphic. The species was multiclonal; however, all populations except one consisted of single multilocus genotypes. Although no viable seeds have been found in the wild, seeds have been produced ex situ by open-pollinated crosses between cultivated accessions. These observations, in conjunction with the allozyme data, suggest that Z. celata is not sexually reproducing in situ because most populations consist of a single selfincompatible individual. If so, the longterm persistence of Z. celata in the wild may require population restoration in the form of providing genotypic diversity that would foster sexual reproduction. The successful ex situ production of seedlings with different multilocus genotypes provides hope for population restoration and for reconstituting the genotypic diversity of this endangered shrub.