Two coexisting tank bromeliads host distinct algal communities on a tropical inselberg.

The tank bromeliads Aechmea aquilega (Salisb.) and Catopsis berteroniana (Schultes f.) coexist on a sun-exposed Neotropical inselberg in French Guiana, where they permit conspicuous freshwater pools to form that differ in size, complexity and detritus content. We sampled the algal communities (both eukaryotic and cyanobacterial taxa, including colourless forms) inhabiting either A. aquilega (n = 31) or C. berteroniana (n = 30) and examined differences in community composition and biomass patterns in relation to several biotic and abiotic variables. Chlorella sp. and Bumilleriopsis sp. were the most common taxa and dominated the algal biomass in A. aquilega and C. berteroniana, respectively. Using a redundancy analysis, we found that water volume, habitat complexity and the density of phagotrophic protozoa and collector-gatherer invertebrates were the main factors explaining the distribution of the algal taxa among the samples. Hierarchical clustering procedures based on abundance and presence/absence data clearly segregated the samples according to bromeliad species, revealing that the algal communities in the smaller bromeliad species were not a subset of the communities found in the larger bromeliad species. We conclude that, even though two coexisting tank bromeliad populations create adjacent aquatic habitats, each population hosts a distinct algal community. Hence, bromeliad diversity is thought to promote the local diversity of freshwater algae in the Neotropics.

Genetic isolation between two sympatric host plant races of the European corn borer, Ostrinia nubilalis Hubner. II: assortative mating and host-plant preferences for oviposition.

The European corn borer, Ostrinia nubilalis Hubner, colonized maize (Zea mays L.) after its introduction into Europe about 500 years ago and is now considered one of the main pests of this crop. In northern France, two sympatric host races have been described: one feeding on maize and the other on mugwort (Artemisia vulgaris L.) and hop (Humulus lupulus L.). In a previous study, we showed that mating between the two races may be impeded by differences in the timing of moth emergence and in the composition of the sex pheromone produced by the females. In this study, we further investigated the genetic isolation of these two races using strains from the maize (Z strain) and mugwort (E strain) races selected for diagnostic alleles at two allozyme loci. In a cage containing maize and mugwort plants and located in natural conditions, mating between individuals of the same strain occurred more often than mating between males and females of the E and Z strains. In particular, we obtained no evidence for crosses between Z females and E males. We also found that females of the Z strain laid their eggs almost exclusively on maize, whereas females of the E strain laid their eggs preferentially, but not exclusively, on mugwort. These results suggest that the genetic differentiation between the two host races may also be favored by host-plant preference, one of the first steps toward sympatric speciation.

Host-plant specialization in pheromone strains of the European corn borer Ostrinia nubilalis in France.

European corn borer (ECB) feeding on maize (Zea mais), mugwort (Artemisia vulgaris), and hop (Humulus lupulus) are genetically different in France and referred to as host-plant races. Here, we investigated sex pheromone composition as a possible trait linked to the host plant. ECB host races were sampled from 13 different sites in France. GC-MS analysis of female pheromone showed that 175 out of 176 maize females belonged to the Z type with one hybrid. In contrast, mugwort and hop females belonged almost exclusively to the E type. No Z females were found on these plants and only 2 females out of 169 were hybrids. In the three sites of sympatry, the hybrid proportion was far from Hardy-Weinberg expectations. Wind tunnel experiments showed that 76-79% of maize males from three populations were attracted by Z females, whereas neither mugwort nor hop males were. Mugwort males from Toussus-le-Noble were attracted by E females originating from an American maize strain. These data showed that maize, mugwort, and hop host races of O. nubilalis differ not only in their host plant but also in the sex pheromone they use. Because mugwort and hop are putative ancestral host plants, these results are discussed from the point of view of evolutionary scenarios for the emergence of Z and E strains.