THE INFLUENCE OF MATING SYSTEM AND OVERLAPPING GENERATIONS ON EFFECTIVE POPULATION SIZE

The effective population size (N_e) depends strongly on mating system and generation time. These two factors interact such that, under many circumstances, N_e is close to N/2, where N is the number of adults. This is shown to be the case for both simple and highly polygynous mating systems. The random union of gametes (RUG) and monogamy are two simple systems previously used in estimating N_e, and here a third, lottery polygyny, is added. Lottery polygyny, in which all males compete equally for females, results in a lower N_e than either RUG or monogamy! Given nonoverlapping generations the reduction is 33% for autosomal loci and 25% for sex-linked loci. The highly polygynous mating systems, harem polygyny and dominance polygyny, can give very low values of N_e/N when the generation time (T) is short. However, as T is lengthened, N_e approaches N/2. The influence of a biased sex ratio depends on the mating system and, in general, is not symmetrical. Biases can occur because of sex differences in either survival or recruitment of adults, and the potential for a sex-ratio bias to change N_e is much reduced given a survival bias. The number of juveniles present also has some influence: as the maturation time is lengthened, N_e increases.     

On body size and habitat selection in rotifers in a macrophye-dominated lake Budzyńskie,Poland

Lake Budzyńskie is shallow, freshwater lake with a well-developed and differentiated macrophytic vegetation. Zooplankton samples were collected from five stations: two of them in submerged macrophytes (Chara and Myriophyllum), one in the zone of floating leaves (Potamogeton), a rush station (Typha) and one in the open water surrounding the vegetation beds. The mean Rotifera densities differed significantly between the lake parts. Furthermore, different habitats were characterised by differences in body size with the exception of the middle body size group (Keratella cochlearis, Polyarthra vulgaris and Trichocerca similis), which was dominated by limnetic representatives. However, in all the other size-dependent groups both stands of submerged macrophytes were characterised by much higher densities than other zones. Additionally, body size within the examined habitats significantly differed. Thus, the size structure of Rotifera communities was directly related to morphological and spatial structures of the substrata. Two groups of habitats were distinguished: the first one consisting of open water and two vegetated zones of less complicated structure (Potamogeton and Typha), and the second of more complex submerged macrophyte species (Chara and Myriophyllum). The differentiation of the architecture of macrophytes affected the nutritional conditions and refuge effectiveness of these habitats.     

Sex ratio variation among broods of great reed warblers Acrocephalus arundinaceus

The sex of 746 great reed warbler fledglings (from 175 broods) was determined by the use of single primer polymerase chain reaction. The reliability of the technique was confirmed as 104 of the fledglings were subsequently recorded as adults of known sex. The overall sex ratio did not differ from unity. Variation in sex ratios between broods was larger than expected from a binomial distribution. Female identity explained some of the variation of brood sex ratio indicating that certain females consistently produced sex ratios that departed from the average value in the population. The theory of sex allocation predicts that parents should adjust the sex ratio of their brood to the relative value of sons and daughters and this may vary in relation to the quality of the parents or to the time of breeding. In the great reed warbler, the proportion of sons was not related to time of breeding, or to any of five female variables. Of five male variables, males with early arrival date tended to produce more daughters. The sex ratio of fledglings that were a result of extra-pair fertilizations did not differ from that of legitimate fledglings. Hence, there is currently no evidence of that female great reed warblers invest in a higher proportion of sons when mated with attractive males.     

Molecular phylogenetic identification of the intestinal anaerobic microbial community in the hindgut of the termite, Reticulitermes speratus, without cultivation

A termite maintains an anaerobic microbial community in its hindgut, which seems to be the minimum size of an anaerobic habitat. This microbial community consists of bacteria and various anaerobic flagellates, and it is established that termites are totally dependent on the microbes for the utilization of their food. The molecular phylogene-tic diversity of the intestinal microflora of a lower termite, Reticulitermes speratus, was examined by a strategy that does not rely on cultivation of the resident microorganisms. Small subunit ribosomal RNA (ssrRNA) genes were directly amplified from the mixed-population DNA of the termite gut by polymerase chain reaction (PCR) and clonally isolated. Most sequenced clones were phylogenetically affiliated with the four major groups of the domain Bacteria: the Proteobacteria group, the Spirochete group, the Bacteroides group, and the Low G + C gram-positive bacteria. The 16S rRNA sequence data show that the majority of the intestinal microflora of the termite consists of new species that are yet to be cultured. The phylogeny of a symbiotic methanogen inhabiting the gut of a lower termite (R. speratus) was analyzed without cultivation. The nucleotide sequence of the ssrDNA and the predicted amino acid sequence of the mcrA product were compared with those of the known methanogens. Both comparisons indicated that the termite symbiotic methanogen belonged to the order Methanobacteriales but was distinct from the known members of this order. The diversity of nitrogen-fixing organ-isms was also investigated without culturing the resident microorganisms. Fragments of the nifH gene, which encodes the dinitrogenase reductase, were directly amplified from the mixed-population DNA of the termite gut and were clonally isolated. The phylogenetic analysis of the nifH amino acid sequences showed that there was a remarkable diversity of nitrogenase genes in the termite gut. The molecular phylogeny of a symbiotic hypermastigote Trichonympha agilis (class Parabasalia; order Hypermastigida) in the hindgut of R. speratus was also examined by the same strategy. The whole-cell hybridization experiments indicated that the sequence originated from a large hypermastigote in the termite hindgut, Trichonympha agilis. According to the phylogenetic trees constructed, the hypermastigote represented one of the deepest branches of eukaryotes. The hypermastigote along with members of the order Trichomonadida formed a monophyletic lineage, indicating that the hypermastigote and trichomonads shared a recent common ancestry. Received: January 22, 1998 / Accepted: February 16, 1998