Reproductive strategies of epialgal bryozoans

Summary

Bryozoans are common encrusting organisms in many shallow-water marine environments. Although reproducing sexually, their success as space occupiers resides largely in their capacity for colonial growth by zooidal budding (regarded by some as a form of asexual reproduction). This paper examines the reproductive strategies of several bryozoan species commonly associated with the fronds of coastal macroalgae. These range from ephemeral species that grow rapidly, reproduce and die (Electra pilosa, Celleporella hyalina, Membranipora membranacea) to more or less annual species with well-developed reproductive and growth cycles (Flustrellidra hispida, Alcyonidium spp.). Whilst many of these bryozoans brood relatively few short-lived lecithotrophic larvae, others produce large numbers of longer-lived planktotrophic larvae. The seasonal and daily patterns of larval release are described for selected species. Resource allocation to sexual and colonial functions is considered in the context of environment-genotype interactions. Reproductive strategy is especially important in determining dispersal and colonising ability, and these in turn are major determinants of ecological pattern in epialgal bryozoan communities.     

Is realised connectivity among populations of aquatic fauna predictable from potential connectivity?

1. Effective management of aquatic fauna requires knowledge of the ways in which populations in different catchments and sub‐catchments are connected. A powerful way to estimate this is using genetic markers, which provide information on the average amount of genetic connectivity among populations over generations. Although many studies of genetic connectivity have appeared in the literature, there are innumerable species that have not been studied. 2. This study explores whether it is possible to make broad generalisations about population connectivity, based on readily available information in the form of species life history and architecture of the aquatic habitat. 3. A number of models have been proposed to explain the pattern of connectivity shown by aquatic species with different life‐history characteristics, for example, the stream hierarchy model, Isolation by Distance, the Death Valley Model, the headwater model and panmixia. 4. In this study, we propose a dichotomous key to assign species to different models of potential connectivity. The key is based on a few very simple questions about the life history of the species and the geographical arrangement of study sites. We then assessed the performance of the key with 109 data sets of Australian fish and macroinvertebrates, using genetic data to provide an estimate of realised connectivity. 5. The realised connectivity fitted the proposed potential connectivity model in over 70% of cases, and we suggest this might be a useful initial approach for managers where empirical data are lacking.     

Microfossils and evidence of land plant evolution

Resolving the question of (1) land plant and (2) vascular land plant evolution is a complex problem. It requires a continued, multidisciplinary effort. We are all grateful to Dianne Edwards and her colleagues (Edwards et al. 1979) for pursuing one such line of research with careful attention to morphology and stratigraphy. Their efforts, however, are confined to one small part of the globe, and they perforce involve only one type of evidence effective with regard to questions of vascular plant evolution once the land estate has been reached. They do not deal with the basic question of vascular and non-vascular land plant origins. The so-called Přídolí benchmark that remained the sine qua nun for vascular plant evolution when Dianne Edwards began her studies has now been irrevocably breached. What other supposedly sacrosanct benchmarks will also crumble as the result of future discoveries, now that vascular plants have been recovered from lower Ludlovian strata? Nevertheless, pre-Devonian plant megafossils are too rare to yield definitive results with regard to the origin of land plants prior to the vascular estate, even though they could in principle (as emphasized by Edwards et al. 1979) provide the least ambiguous evidence. We must look elsewhere than to megafossils for evidence suggesting links in time between green algal ancestors, major groups of living green land Plants, and the many extinct groups of enigmatic higher land plants in the crucial Early Paleozoic time interval.     

Interspecific competition in Drosophila II. Competitive outcome in some 2–resource environments

Populations of Drosophila metanogaster and D. simulans were made to compete in a number of 2– resource environments. The resources used were all based on a standard Drosophila medium and differed from each other only in their concentrations of ethanol. Each experiment comprised three cages started at a species–frequency of 0.8 D. melanogaster and three at 0.2 D. melanogaster. This enabled screening for a wide range of equilibria.
Reversal of competitive superiority was achieved (unintentionally) by raising the alcohol level of one of the resources. Thus while D. simulans won in an environment with media containing 0 and 896 ethanol, D. melanogaster won if the ethanol concentrations were 0 and 10%, or 0 and 12%. This parallels the reversal of dominance reported earlier for single–resource systems. However, in none of the 2–resource systems studied was there a stable equilibrium in species–frequency–in fact there was no significant frequency–dependence at all.
An artificial selection experiment produced a stock of D. melanogaster ovipositing a higher proportion of eggs on the alcoholic resource. Thus, this stock was more divergent from D. simulans , in at least one aspect of resource use, than was the original stock of D. melanogaster. However, competition between stock D. simulans and selected D. melanogaster showed no frequency–dependence. Again, reversal of competitive dominance was unintentionally achieved.
The implications of these results are discussed, and the need for a comprehensive study of resource–utilization in a pair of competing species is stressed.     

Changes in Nuclear DNA Content, Cell and Nuclear Size, and Frequency of Cell Division in the Cotyledons of Brassica napus L. during Embryogenesis

Cellular behaviour was examined during embryogenesis in Brassicanapus to test whether or not polyploidy occurs in the cotyledonsduring the phase of oil deposition. Nuclear DNA content, nuclearand cell size, and the mitotic index were measured in the cotyledonson various days post anthesis (dpa). In squashed monolayersfrom 15 dpa cotyledons, a polyploid (>5C) population wasdetected together with a substantial number of cells in G2 (4C).Nuclear volume was measured on sectioned tissues and, at 15dpa, the range of values from the cotyledons (40–500 *m³)contrasted with that in the vestigial suspensor and endosperm(50–> 600 µm³). At 15 dpa the nuclear volumedata suggest that whilst cells in the cotyledons were in Gland G2 many endosperm and suspensor cells were polyploid. Thus,polyploidy observed in the squashed monolayers was probablydue to contaminating endosperm/suspensor cells. At 25 and 35dpa, polyploidy was not detected; all cells were in Gl (2C)and cell area increased. The mitotic index peaked at 20 dpabefore declining and given the narrower distribution of nuclearvolumes at 25 and 35 dpa (50–300 µm³), these dataare consistent with cell arrest in Gl. Thus, polyploidy wasnot detected in the cotyledons of B. napus which differs fromwhat is known about cellular development in legume cotyledons. Key words: Brassica napus L., DNA, nuclear volume     

Novel fungal disease in complex leaf‐cutting ant societies

Abstract 1. The leaf‐cutting ants practise an advanced system of mycophagy where they grow a fungus as a food source. As a consequence of parasite threats to their crops, they have evolved a system of morphological, behavioural, and chemical defences, particularly against fungal pathogens (mycopathogens). 2. Specific fungal diseases of the leaf‐cutting ants themselves have not been described, possibly because broad spectrum anti‐fungal defences against mycopathogens have reduced their susceptibility to entomopathogens. 3. Using morphological and molecular tools, the present study documents three rare infection events of Acromyrmex and Atta leaf‐cutting ants by Ophiocordyceps fungi, agenus of entomopathogens that is normally highly specific in its host choice. 4. As leaf‐cutting ants have been intensively studied, the absence of prior records of Ophiocordyceps suggests that these infections may be a novel event and that switching from one host to another is possible. To test the likelihood of this hypothesis, host switching was experimentally induced, and successfully achieved, among five distinct genera of ants, one of which was in a different sub‐family than the leaf‐cutter ants. 5. Given the substantial differences among the five host ants, the ability of Ophiocordyceps to shift between such distant hosts is remarkable; the results are discussed in the context of ant ecological immunology and fungal invasion strategies.     

Extensive intraspecific genetic diversity of a freshwater crayfish in a biodiversity hotspot

1. The freshwater crayfish Cherax dispar (Decapoda: Parastacidae) inhabits coastal regions and islands of South East Queensland, Australia. We hypothesised that populations of C. dispar on different islands would be more genetically divergent from each other than populations from different drainages within the same island or on the mainland. 2. Phylogenetic and phylogeographic analyses were conducted on two mitochondrial genes (cytochrome oxidase subunit I & 16S ribosomal DNA) and one nuclear gene (Internal Transcribed Spacer region 2). Phylogeographic patterns were compared with those for other freshwater organisms in the area. 3. Deep genetic divergences were found within C. dispar, including four highly divergent (up to 20%) clades. The geographic distribution of each of the clades revealed strong latitudinal structuring along the coast rather than structuring among the islands. The high genetic divergence observed among the C. dispar clades was estimated to have pre‐dated island formation and may represent ancient river drainage patterns. 4. A restricted distribution was observed for the most divergent clade, which was discovered only on two of the sand islands (North Stradbroke Island and Moreton Island). Furthermore, strong phylogeographic structuring was observed within this clade on North Stradbroke Island, where no haplotypes were shared between samples from opposite sides of the island. This low connectivity within the island supports the idea that C. dispar rarely disperse terrestrially (i.e. across watersheds).