Horn polyphenism and related head shape variation in a single-horned dung beetle: Onthophagus (Palaeonthophagus) fracticornis (Coleoptera: Scarabaeidae)

Horns of Onthophagus beetles are typical examples of phenotypically plastic traits: they are expressed as a function of environmental (nutritional) stimuli, and their reaction norm (i.e. the full set of horn lengths expressed as a response to different degrees of nutritional states) can be either linear or threshold-dependent. Horned males of Onthophagus ( Palaeonthophagus ) fracticornis (Preyssler, 1790) bear a single triangular cephalic protrusion of vertex carina, which has received phylogenetic support as the most primitive horn shape in the genus. Inter- and intra-sexual patterns of horn expression were studied in O. fracticornis by means of static allometries while associated variations in head shape were assessed using geometric morphometric techniques. The relation between log-transformed measurements of body size and vertex carina supported an isometric scaling in females. On the contrary, a sigmoidal model described better the horn length-body size allometry in males, with a switch point between alternative morphs at a pronotum width of 3.88 mm. Sigmoidal static allometries of horns in Onthophagus populations arise from a threshold-dependent developmental process of horn growth. This process underlies the expression of both plesiomorphic and apomorphic horn shapes in the genus. Given that the single-horn model has been identified as primitive, we propose that such a developmental process giving rise to it may be evolutionarily ancient as well. Horn expression was accompanied by a deformation of the head which makes minor and major morphs appear even more different. Therefore, in this species both horn and head shape expression contribute to male dimorphism.     

透明度胁迫对菹草(Potamogeton crispus)生长的定量影响及其生长动力学模型

在实验室研究了不同水体透明度对沉水植物菹草生长的影响,结果表明,在实验条件下,当水体透明度低于12cm时,菹草由于水下光照不足而无法存活;透明度为15cm时菹草的生物量明显低于透明度为70cm(对照)、35cm和20cm 3种情况下的生物量,表明在透明度为15cm时,菹草的生长受到了明显的抑制.实验期间,菹草生物量变化的回归分析表明,不同透明度下菹草的生长均符合逻辑斯谛曲线.采用逻辑斯谛方程描述菹草的生长规律,结合实验结果,确定不同水体透明度下菹草的生长参数,结果显示:水体透明度对菹草生长的内禀增长率rm影响较大,而对菹草生长的环境容纳量k无明显影响.综合考虑水体透明度对菹草生长的影响,建立考虑水深和透明度因素的菹草生长动力学模型,该模型可预测出在外部水环境因素作用下菹草的动态生长过程.     

Promising directions in plant phenotypic plasticity

A research agenda for the next phase of plasticity studies calls for contributions from a diverse group of biologists, working both independently and collaboratively, to pursue four promising directions: examining dynamic, anatomical/architectural, and cross-generational plasticity along with simpler growth traits; carefully assessing the adaptive significance of those plasticity patterns; investigating the intricate transduction pathways that lead from environmental signal to phenotypic response; and considering the rich environmental context of natural systems. Progress in these areas will allow us to address broad and timely questions regarding the ecological and evolutionary significance of plasticity and the nature of phenotypic determination.     

HETEROCHRONIC DEVELOPMENTAL PLASTICITY IN LARVAL SEA URCHINS AND ITS IMPLICATIONS FOR EVOLUTION OF NONFEEDING LARVAE

Preexisting developmental plasticity in feeding larvae may contribute to the evolutionary transition from development with a feeding larva to nonfeeding larval development. Differences in timing of development of larval and juvenile structures (heterochronic shifts) and differences in the size of the larval body (shifts in allocation) were produced in sea urchin larvae exposed to different amounts of food in the laboratory and in the field. The changes in larval form in response to food appear to be adaptive, with increased allocation of growth to the larval apparatus for catching food when food is scarce and earlier allocation to juvenile structures when food is abundant. This phenotypic plasticity among full siblings is similar in direction to the heterochronic evolutionary changes in species that have greater nutrient reserves within the ova and do not depend on particulate planktonic food. This similarity suggests that developmental plasticity that is adaptive for feeding larvae also contributes to correlated and adaptive evolutionary changes in the transition to nonfeeding larval development. If endogenous food supplies have the same effect on morphogenesis as exogenous food supplies, then changes in genes that act during oogenesis to affect nutrient stores may be sufficient to produce correlated adaptive changes in larval development.     

Craniometrical variability and developmental stability. Two useful tools for assessing the population viability of Eurasian otter (Lutra lutra) populations in Europe.

Morphometrical univariate analyses of otter skulls collected over the past hundred years in European countries from presumed healthy populations were compared with skulls from presumed endangered populations. The average degree of sexual dimorphism of die European populations was found to be directly correlated to die skull size of die male otters. Fluctuating asymmetry (FA) in metric skull traits was analysed as an estimator of developmental stability. There was evidence for increased FA in different traits over time in some of the presumed endangered populations, and for a reduction in size of skull traits. In contrast, the healthy populations did not show any significant changes in the same traits during the same period. The reduced sexual dimorphism of the endangered populations is suggested to be a product of relaxed sexual selection and deteriorated habitat conditions. Environmental and genetic forces that may have shaped these patterns are discussed.     

Optimality and phenotypic plasticity of shoot-to-root ratio under variable light and nutrient availabilities

We studied the phenotypic plasticity of shoot-to-root ratio with a model of plant growth in different availabilities of light and nutrients. Optimal shoot-to-root ratio was defined as the equal limitation of growth by light and nutrients. An optimally growing plant had a curved relative growth rate (RGR) isoclines and a faster growth rate than a fixed-allocation plant having right-angled RGR isoclines. We assumed the plant be exposed to a unit standard deviation of bivariate normally distributed resources. Plants were more plastic in a low than in a high resource availability. Negative correlation between resources increased and positive correlation decreased plasticity. Plasticity was high in plants that saturate at low resource availabilities but independent of maximum growth rate. A trade-off between the maximum growth rate and plasticity of shoot-to-root allocation may rise indirectly from the tendency of fast-growing plants to have high resource requirements.     

Locusts use dynamic thermoregulatory behaviour to optimize nutritional outcomes

Because key nutritional processes differ in their thermal optima, ectotherms may use temperature selection to optimize performance in changing nutritional environments. Such behaviour would be especially advantageous to small terrestrial animals, which have low thermal inertia and often have access to a wide range of environmental temperatures over small distances. Using the locust, Locusta migratoria, we have demonstrated a direct link between nutritional state and thermoregulatory behaviour. When faced with chronic restrictions to the supply of nutrients, locusts selected increasingly lower temperatures within a gradient, thereby maximizing nutrient use efficiency at the cost of slower growth. Over the shorter term, when locusts were unable to find a meal in the normal course of ad libitum feeding, they immediately adjusted their thermoregulatory behaviour, selecting a lower temperature at which assimilation efficiency was maximal. Thus, locusts use fine scale patterns of movement and temperature selection to adjust for reduced nutrient supply and thereby ameliorate associated life-history consequences.