Defensive Behavior in Primary- and Secondary-Host Generations of the Soldier-Producing Aphid, Pemphigus bursarius (Hemiptera: Aphididae)

The genus Pemphigus comprises several species that produce soldiers (defensive morphs) in galls on the primary host. At the moment, it is unclear if host-alternating species also produce defenders on their secondary host. We therefore examined how P. bursarius morphs of the secondary host generations respond to predators to test whether they show defensive behavior. We further examined how this response compares with the antipredator behavior of soldiers in P. bursarius and P. spyrothecae occurring on the primary host. We performed two manipulative experiments using two predatory species to quantify the behavior of the different morphs in response to predators. In both experiments, secondary host morphs of P. bursarius showed no attacking behavior and antipredator behavior in these morphs was limited to escaping natural enemies by walking away. In contrast, the first instars of the primary host generations in both Pemphigus species showed attacking behavior and were capable of killing the predators.     

Priority areas of intraspecific diversity: Larzac, a global hotspot for facultative paedomorphosis in amphibians

Although the designation of biodiversity hotspots is a valuable tool to improve conservation efforts, this is a concept primarily based on species diversity. In consequence, another component of biodiversity, adaptive variation, is often ignored in conservation and to my knowledge no attempt has been made to identify hotspots of remarkable intraspecific patterns. My aim was to focus on the process of facultative paedomorphosis (i.e. the retention of larval traits such as gills in adult variants), a rare developmental pathway. One hundred and seventy-four ponds were inventoried in Larzac (France) to determine the distribution and abundance of paedomorphic palmate newts Triturus helveticus (Amphibia, Caudata) and to compare these results with the current distribution of paedomorphs in this and other species. During this study, paedomorphic newts were found in 46 ponds, 32 of which were described here for the first time. Seventy-nine per cent of known paedomorphic populations of this species were found there, whereas this area covers only 0.5% of the distribution area of the species. This represents the highest known density of facultatively paedomorphic populations, all species being considered. Because these populations face a high threat of disappearance, Larzac should be designated as an intraspecific biodiversity hotspot in order to protect adaptive intraspecific variation. Future conservation-oriented work should focus not only on species distributions but also on phenotypically diverse but spatially localized variation.     

Allometric plasticity in a polyphenic beetle

Abstract 1. Environmental conditions, such as variation in nutrition, commonly contribute to morphological variation among individuals by affecting body size and the expression of certain morphological traits; however the scaling relationship between a morphological trait and body size over a range of body sizes is generally assumed not to change in response to environmental fluctuation (allometric plasticity), but instead to be constant and diagnostic for a particular trait and species or population. The work reported here examined diet‐induced allometric plasticity in the polyphenic beetle Onthophagus taurus Schreber (1759) (Coleoptera: Scarabaeidae). 2. Male O. taurus vary in body size depending on larval nutrition. Only males above a critical body size threshold express fully developed horns; males smaller than this threshold develop only rudimentary horns or no horns at all. 3. Field populations that naturally utilise two different resources for feeding larvae (horse dung vs. cow manure) exhibited significant differences in the average scaling relationship between body size and male horn length over the same range of body sizes. Males collected from cow manure populations expressed consistently longer horns for a given body size than males collected from horse dung populations. 4. Males reared in the laboratory on horse dung or cow manure also exhibited significant differences in the average scaling relationship between body size and horn length. Differences between laboratory populations reared on horse dung or cow manure were of the same kind and magnitude as differences between field populations that utilise these different resources naturally. 5. These findings suggest that between‐population differences in scaling relationships between horn length and body size can be the product of differences in the quality of resources available to developing larvae. Results are discussed in the context of onthophagine mating systems and recent insights in the developmental and endocrine control of horn polyphenisms.     

Phase-related body-color polyphenism in hatchlings of the desert locust, Schistocerca gregaria: re-examination of the maternal and crowding effects

The mechanism controlling the body color of hatchlings was studied for the desert locust, Schistocerca gregaria. A pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods has been suggested to cause darkening in their progeny. We re-examined the role of this maternal factor by washing or separating eggs at deposition. Eggs produced by crowd-reared female adults were washed with saline or separated individually without being washed immediately after deposition and the body color of the hatchlings from them was compared with that from the eggs unwashed and kept in the egg pod until hatching. Most hatchlings were dark and no significant difference was found in the proportions of dark- and light-colored hatchlings between the treatments and controls. Likewise, eggs separated before the foam plug deposition produced dark-colored hatchlings as in the un-separated controls. These results demonstrated that neither washing nor separation of eggs at deposition affected the hatchling body coloration. The variation in hatchling body color was correlated closely to the body weight at hatching, indicating that hatchling body color had been determined maternally. Green hatchlings reared under crowded conditions remained green until the second stadium at which black patterns were induced. It was concluded that body color at hatching has been determined maternally and crowding during the first nymphal stadium influences nymphal body color but its effect is not manifested until the second stadium. The present study casts doubts on the presence of a recently suggested pheromonal factor on the color of the hatchlings.     

Dynamics of an ant-ant obligate mutualism: colony growth, density dependence and frequency dependence

In insect societies, worker vs. queen development (reproductive caste) is typically governed by environmental factors, but many Pogonomyrmex seed-harvester ants exhibit strict genetic caste determination, resulting in an obligate mutualism between two reproductively isolated lineages. Same-lineage matings produce fertile queens while alternate-lineage matings produce sterile workers. Because new virgin queens mate randomly with multiple males of each lineage type, and both worker and queen phenotypes are required for colony growth and future reproduction, fitness is influenced by the relative frequency of each lineage involved in the mutualistic breeding system. While models based solely on frequency-dependent selection predict the convergence of lineage frequencies towards equal (0.5/0.5), we surveyed the lineage ratios of 49 systems across the range of the mutualism and found that the global lineage frequency differed significantly from equal. Multiple regression analysis of our system survey data revealed that the density and relative frequency of one lineage decreases at lower elevations, while the frequency of the alternate lineage increases with total colony density. While the production of the first worker cohort is largely frequency dependent, relying on the random acquisition of worker-biased sperm stores, subsequent colony growth is independent of lineage frequency. We provide a simulation model showing that a net ecological advantage held by one lineage can lead to the maintenance of stable but asymmetric lineage frequencies. Collectively, these findings suggest that a combination of frequency-dependent and frequency-independent mechanisms can generate many different localized and independently evolving system equilibria.     

Trophic determinants of hypopus induction in the stored-product mite Lepidoglyphus destructor (Acari: Astigmata)

A unique (synapomorphic) characteristic of astigmatic mites is the heteromorphic deuteronymph also called hypopus. It is a non-feeding and facultative instar between protonymph and tritonymph. The hypopus is adapted for dispersal and sometimes also for dormancy, as in Lepidoglyphus destructor. The experiments reveal a correlation between the composition of the foodstuff, the duration of development of homomorphic instars, the mortality of protonymphs and the production of hypopodes. As food quality decreases, development lasts longer, mortality increases and hypopodes are produced in greater numbers. Disadvantageous trophic conditions of varied chemical nature favour the induction of hypopodes. The experimental data show that hypopus incidences (as percentage individuals of a population) depend on the relative proportions of constituents of an ingested foodstuff. What matters is the ratio between nourishing foodstuff components and those that are of little or no nutritional value. When a certain ratio does not meet a presumed metabolically required level of nutrients a nutritional deficiency results and hypopus induction is triggered, provided that adequate genetic propensities for hypopus production are present (L. destructor is highly polymorphic for hypopus production). Specific key substances are apparently not involved, and composite properties of a foodstuff are crucial for hypopus induction. Decrease of food quality (not poor food per se) during the hypopus-inducible period (late larval to early protonymphal phase) promotes hypopus induction. The interpretation matches the ecological scene. When trophic deterioration of a patch habitat sets in, often as a result of overcrowding, conditions will eventually become untenable. As a response to incurring nutritional deficiencies the mites will induce hypopodes, which provide for escape from or survival at the decaying habitat patch. Experiments support the threshold model of quantitative genetics for hypopus expression as previously inferred from other experiments with L. destructor.     

Testing the status-dependent ESS model: population variation in fighter expression in the mite Sancassania berlesei

The conditional evolutionarily stable strategy (ESS) with status-dependent tactics is the most commonly invoked ESS for alternative reproductive tactics within the sexes. Support for this model has recently been criticized as apparent rather than real. We address key predictions of the status-dependent ESS in three populations of the male dimorphic mite Sancassania berlesei. In S. berlesei'fighter' males are characterized by a thickened pair of legs used for killing rivals; 'scramblers' are benign. Most males in each population could be manipulated to become fighters by decreasing density, fulfilling the prediction that males make a 'decision'. There was evidence of genetic covariance between sire status and offspring morph, but also a strong effect of sire morph on offspring morph ratio. This was consistent with considerable genetic variation for the status-dependent switch point as a breeding experiment found no support for single-locus inheritance. We also found evidence that switch points evolve independently of distributions of status. This study supports the current status-dependent ESS model.