Identifying biochemical phenotypic differences between cryptic species

Molecular genetic methods can distinguish divergent evolutionary lineages in what previously appeared to be single species, but it is not always clear what functional differences exist between such cryptic species. We used a metabolomic approach to profile biochemical phenotype (metabotype) differences between two putative cryptic species of the earthworm Lumbricus rubellus. There were no straightforward metabolite biomarkers of lineage, i.e. no metabolites that were always at higher concentration in one lineage. Multivariate methods, however, identified a small number of metabolites that together helped distinguish the lineages, including uncommon metabolites such as Nε-trimethyllysine, which is not usually found at high concentrations. This approach could be useful for characterizing functional trait differences, especially as it is applicable to essentially any species group, irrespective of its genome sequencing status.     

Conflicting preferences within females: sexual selection versus species recognition

Preferences for mates within and between species are often harmonious, as traits that females prefer are usually more developed in conspecifics than heterospecifics. This need not be the case, however. When it is not, conflict between these arenas of mate choice can be resolved if females attend to different cues for each task. But this raises the potential for correlations among preferences to limit the opportunity for these two processes to operate independently. Here, we show that, within individual female pygmy swordtails (Xiphophorus pygmaeus), directional preferences for conspicuous ornamentation are inversely associated with discrimination against a sympatric heterospecific, Xiphophorus cortezi. Thus, mate choice among and within species need not be separate, independent processes; instead, they can be mechanistically intertwined. As a consequence, different arenas of mate choice can constrain one another, even when females assess multiple cues.     

Interspecific aggression and character displacement in the damselfly Calopteryx splendens

Problems in species recognition are thought to affect the evolution of secondary sexual characters mainly through avoidance of maladaptive hybridization. Another, but much less studied avenue for the evolution of sexual characters due to species recognition problems is through interspecific aggression. In the damselfly, Calopteryx splendens, males have pigmented wing spots as a sexual character. Large-spotted males resemble males of another species, Calopteryx virgo, causing potential problems in species recognition. In this study, we investigate whether there is character displacement in wing spot size and whether interspecific aggression could cause this pattern. We found first that wing spot size of C. splendens in populations decreased with increasing relative abundance of C. virgo. Secondly, C. virgo males were more aggressive towards large- than small-spotted C. splendens males. Thirdly, in interspecific contests C. virgo males had better territory holding ability than C. splendens males. These results suggest that interspecific aggression may have caused character displacement in wing spot size of C. splendens, because the intensity of aggression towards large-spotted males is likely to increase with relative abundance of C. virgo males. Thus, interspecific aggression may be an evolutionarily significant force that is able to cause divergence in secondary sexual characters.     

Reciprocal feeding facilitation between above- and below-ground herbivores

Interspecific interactions between insect herbivores predominantly involve asymmetric competition. By contrast, facilitation, whereby herbivory by one insect benefits another via induced plant susceptibility, is uncommon. Positive reciprocal interactions between insect herbivores are even rarer. Here, we reveal a novel case of reciprocal feeding facilitation between above-ground aphids (Amphorophora idaei) and root-feeding vine weevil larvae (Otiorhynchus sulcatus), attacking red raspberry (Rubus idaeus). Using two raspberry cultivars with varying resistance to these herbivores, we further demonstrate that feeding facilitation occurred regardless of host plant resistance. This positive reciprocal interaction operates via an, as yet, unreported mechanism. Specifically, the aphid induces compensatory growth, possibly as a prelude to greater resistance/tolerance, whereas the root herbivore causes the plant to abandon this strategy. Both herbivores may ultimately benefit from this facilitative interaction.     

Character displacement in the fighting colours of Hetaerina damselflies

Aggression between species is a seldom-considered but potentially widespread mechanism of character displacement in secondary sexual characters. Based on previous research showing that similarity in wing coloration directly influences interspecific territorial aggression in Hetaerina damselflies, we predicted that wing coloration would show a pattern of character displacement (divergence in sympatry). A geographical survey of four Hetaerina damselfly species in Mexico and Texas showed evidence for character displacement in both species pairs that regularly occurs sympatrically. Hetaerina titia, a species that typically has large black wing spots and small red wing spots, shifted to having even larger black spots and smaller red wing spots at sites where a congener with large red wing spots is numerically dominant (Hetaerina americana or Hetaerina occisa). Hetaerina americana showed the reverse pattern, shifting towards larger red wing spots where H. titia is numerically dominant. This pattern is consistent with the process of agonistic character displacement, but the ontogenetic basis of the shift remains to be demonstrated.     

Population regulation of territorial species: both site dependence and interference mechanisms matter

Spatial patterns of site occupancy are commonly driven by habitat heterogeneity and are thought to shape population dynamics through a site-dependent regulatory mechanism. When examining this, however, most studies have only focused on a single vital rate (reproduction), and little is known about how space effectively contributes to the regulation of population dynamics. We investigated the underlying mechanisms driving density-dependent processes in vital rates in a Mauritius kestrel population where almost every individual was monitored. Different mechanisms acted on different vital rates, with breeding success regulated by site dependence (differential use of space) and juvenile survival by interference (density-dependent competition for resources). Although territorial species are frequently assumed to be regulated through site dependence, we show that interference was the key regulatory mechanism in this population. Our integrated approach demonstrates that the presence of spatial processes regarding one trait does not mean that they necessarily play an important role in regulating population growth, and demonstrates the complexity of the regulatory process.     

Germination traits explain soil seed persistence across species: the case of Mediterranean annual plants in cereal fields

Background and Aims

Seed persistence in the soil under field conditions is an important issue for the maintenance of local plant populations and the restoration of plant communities, increasingly so in the light of rapidly changing land use and climate change. Whereas processes important for dispersal in space are well known, knowledge of processes governing dispersal in time is still limited. Data for morphological seed traits such as size have given contradictory results for prediction of soil seed persistence or cover only a few species. There have been few experimental studies on the role of germination traits in determining soil seed persistence, while none has studied their predictive value consistently across species. Delayed germination, as well as light requirements for germination, have been suggested to contribute to the formation of persistent seed banks. Moreover, diurnally fluctuating temperatures can influence the timing of germination and are therefore linked to seed bank persistence.

Methods

The role of germination speed measured by T₅₀ (days to germination of 50 % of all germinated seeds), light requirement and reaction to diurnally fluctuating temperatures in determining seed persistence in the soil was evaluated using an experimental comparative data set of 25 annual cereal weed species.

Key Results

It is shown that light requirements and slow germination are important features to maintain seeds ungerminated just after entering the soil, and hence influence survival of seeds in the soil. However, the detection of low diurnally fluctuating temperatures enhances soil seed bank persistence by limiting germination. Our data further suggest that the effect of diurnally fluctuating temperatures, as measured on seeds after dispersal and dry storage, is increasingly important to prevent fatal germination after longer burial periods.

Conclusions

These results underline the functional role of delayed germination and light for survival of seeds in the soil and hence their importance for shaping the first part of the seed decay curve. Our analyses highlight the detection of diurnally fluctuating temperatures as a third mechanism to achieve higher soil seed persistence after burial which interacts strongly with season. We therefore advocate focusing future research on mechanisms that favour soil seed persistence after longer burial times and moving from studies of morphological features to exploration of germination traits such as reaction to diurnally fluctuating temperatures.     

A natural genetic polymorphism affects retroactive interference in Drosophila melanogaster

As environments change, animals update their internal representations of the external world. New information about the environment is learned and retained whereas outdated information is disregarded or forgotten. Retroactive interference (RI) occurs when the retrieval of previously learned information is less available owing to the acquisition of recently acquired information. Even though RI is thought to be a major cause of forgetting, its functional significance is still under debate. We find that natural allelic variants of the Drosophila melanogaster foraging gene known to affect rover and sitter behaviour differ in RI. More specifically, rovers who were previously shown to experience greater environmental heterogeneity while foraging display RI whereas sitters do not. Rover responses are biased towards more recent learning events. These results provide an ecological context to investigate the function of forgetting via RI and a suitable genetic model organism to address the evolutionary relevance of cognitive tasks.     

Experimental evidence that competition between stickleback species favours adaptive character divergence

The ecological character displacement hypothesis assumes that the effects of interspecific resource competition cause divergent selection to favour phenotypes that exploit non-shared resources. This model predicts that interspecific competition declines with increased divergence. Direct tests of this decline are rare despite much comparative evidence for character displacement. We tested this prediction using a pair of divergent brook sticklebacks. Brook sticklebacks sympatric with ninespine sticklebacks have diverged from local allopatric brook populations, and so the two types of brook sticklebacks potentially represent pre- and post-displacement forms. We used enclosures placed in a lake to compare short-term fitness (growth) of sympatric (post-displacement) and allopatric (pre-displacement) brook forms in the presence and absence of ninespine sticklebacks. Brook sticklebacks grew less in the presence vs. absence of ninespine sticklebacks, indicating that interspecific competition occurred. As expected, allopatric brook forms had lower growth than sympatric forms when ninespine sticklebacks were present. This result suggests that ecological character displacement has occurred.     

A review of Rheocricotopus (Psilocricotopus) chalybeatus species group from China,with the description of three new species (Diptera,Chironomidae)

The Rheocricotopus (Psilocricotopus) chalybeatus species group from China is reviewed. Three new species, R. (P.) brochus sp. n., R. (P.) rotundus sp. n. and R. (P.) serratus sp. n. are described as adult males. R. (P.) imperfectus Makarchenko & Makarchenko, 2005, R. (P.) robacki (Beck & Beck, 1964) and R. (P.) valgus Chaudhuri & Sinharay, 1983 are recorded from China for the first time and annotated. The diagnosis for the species group is emended and a key to adult males of the species group in China is presented.     

Cytogenetic characterization of two species of Frieseomelitta Ihering, 1912 (Hymenoptera, Apidae, Meliponini)

The cytogenetic analysis of Frieseomelitta dispar and F. francoi revealed the chromosome numbers 2n = 30 and n = 15 and a karyotypic formula 2K = 4M+2M(t)+4A+20A(M). The number of chromosomes observed was consistent with those reported for other Frieseomelitta species. The occurrence of the M(t) chromosome and other features of the karyotype formulae suggest a close relationship between F. dispar, F. francoi and F. varia. Nevertheless, it was possible to differentiate the karyotypes of the species by DAPI/CMA(3) staining, which revealed GC-rich regions on two chromosome pairs of F. dispar: one acrocentric and one pseudoacrocentric. In F. francoi, the same kinds of regions were observed on a pair of metacentrics and on a pair of acrocentrics. Our analysis also confirmed the chromosome number conservation in Frieseomelitta and suggests that infrequent pericentric inversion could constitute a synapomorphy for the group including F. dispar, F. francoi, and F. varia.