Fruit to flower ratios and trade-offs in size and number

Summary Hermaphroditic flowering plants commonly produce many more flowers than fruits. To interpret this observation, I suggest an explanation based on trade-offs occurring between the size and the number of flowers and fruits that mature when resources are limiting. I make this suggestion concrete with the aid of a phenotypic model analysed using the ESS approach. The model includes resource allocations to male and female reproductive structures at the time of flowering and to fruit maturation. The formulation allows for non-linear relations between opportunities for fertility gain on the one hand and flower and fruit size and number on the other. Results of the model indicate that fewer fruits will be matured than flowers produced, even in the absence of previously suggested factors such as pollinator limitation and bet-hedging in the face of environmental variability. Model results emphasize the distinction between flower versus inflorescence contributions to male and female reproductive success. Analysis also shows that the fruit to flower ratio in hermaphroditic plants will be lower than that in female plants, in accord with broad taxonomic surveys. Finally, results obtained lead to empirically testable predictions about the relationship between flower size, fruit size and the fruit to flower ratio. Methods to test these predictions are discussed.     

Evolution of floral display in Eichhornia paniculata (Pontederiaceae): genetic correlations between flower size and number

The evolution of floral display is thought to be constrained by trade‐offs between the size and number of flowers and inflorescences. We grew in the glasshouse 60 maternal families from each of two Brazilian populations of the annual herb, Eichhornia paniculata. We measured flower size, daily flower number, and total flower number per inflorescence, and two indices of module size, leaf area and age at flowering. We also assessed the size and number of inflorescences produced over 6 weeks. All floral traits exhibited significant heritable variation, some of which was due to genetic variation in module size. Genetic (maternal family) correlations between daily and total flower number did not differ from 1.0, indicating that display size (daily flower number) cannot evolve independently from total flower number per inflorescence. Genetic correlations between flower size and daily flower number ranged from negative to positive (r=–0.78 to +0.84), depending on population and inflorescence. Positive correlations occurred when variation in investment per inflorescence was high so that some families produced both larger and more flowers. These correlations became zero when we controlled for variation in module size. Families that flowered later produced fewer, larger inflorescences (r=–0.33, –0.85). These data support theoretical predictions regarding the combined effects of variation in resource acquisition and allocation on traits involved in trade‐offs, and they emphasize the hierarchical organization of floral displays. Our results imply that patterns of resource allocation among inflorescences influence evolutionary changes in flower size and number per inflorescence.     

Parent–offspring conflict and selection on egg size in turtles

The trade‐off between offspring size and number can present a conflict between parents and their offspring. Because egg size is constrained by clutch size, the optimal egg size for offspring fitness may not always be equivalent to that which maximizes parental fitness. We evaluated selection on egg size in three turtle species (Apalone mutica, Chelydra serpentina and Chrysemys picta) to determine if optimal egg sizes differ between offspring and their mothers. Although hatching success was generally greater for larger eggs, the strength and form of selection varied. In most cases, the egg size that maximized offspring fitness was greater than that which maximized maternal fitness. Consistent with optimality theory, mean egg sizes in the populations were more similar to the egg sizes that maximized maternal fitness, rather than offspring fitness. These results provide evidence that selection has maximized maternal fitness to achieve an optimal balance between egg size and number.     

A genotypic trade‐off between the number and size of clonal offspring in the stoloniferous herb Potentilla reptans

A negative, genetic correlation between the total number and average size of progeny is a classical life‐history trade‐off that can greatly affect the fitness of organisms in their natural environments. This trade‐off has been investigated for animals and for sexually reproducing plants. However, evidence for a genetical size‐number trade‐off for clonal progeny in plants is still scarce. This study provides experimental evidence for such a trade‐off in the stoloniferous herb Potentilla reptans, and it studies phenotypic plasticity to light availability for the involved traits. Genotypes of P. reptans were collected from distinctively different environments, clonally replicated and exposed to high light and to shaded conditions. We found a significant negative correlation between the average size and the total number of offspring across genotypes for both light environments. Shading reduced ramet numbers, but hardly affected average ramet size.     

One big, and many small reasons that direct selection on offspring number is still open for discussion

In a recent paper, I proposed that natural selection should act to increase offspring number when diversification bet hedging is favoured. The simple underlying reasoning is that a target diversification strategy is more reliably generated with increasing sample size. The intention of opening a discussion has been realized; recent criticisms of the idea argue that selection does not act to increase offspring number when population size is large or infinite. Here I agree that criticisms have merit; indeed they are largely confined to the caveats discussed in my original paper. The critique, however, implies a verdict of outright rejection of the idea of selection on offspring number, which would be erroneous. Contrary to the assertions of the criticism, then, the importance of selection acting directly on offspring number remains an open question.     

Threshold criteria for conversion of probability of species presence to either–or presence–absence

For many applications the continuous prediction afforded by species distribution modeling must be converted to a map of presence or absence, so a threshold probability indicative of species presence must be fixed. Because of the bias in probability outputs due to frequency of presences (prevalence), a fixed threshold value, such as 0.5, does not usually correspond to the threshold above which the species is more likely to be present. In this paper four threshold criteria are compared for a wide range of sample sizes and prevalences, modeling a virtual species in order to avoid the omnipresent error sources that the use of real species data implies. In general, sensitivity–specificity difference minimizer and sensitivity–specificity sum maximizer criteria produced the most accurate predictions. The widely-used 0.5 fixed threshold and Kappa-maximizer criteria are the worst ones in almost all situations. Nevertheless, whatever the criteria used, the threshold value chosen and the research goals that determined its choice must be stated.     

Quality–quantity trade‐off of human offspring under adverse environmental conditions

A central paradigm in life‐history theory is the trade‐off between offspring number and quality. Several studies have investigated this trade‐off in humans, but data are inconclusive, perhaps because prosperous socio‐cultural factors mask the trade‐off. Therefore, we studied 2461 offspring groups in an area under adverse conditions in northern Ghana with high fertility and mortality rates. In a linear mixed model controlling for differences in age and tribe of the mother and socioeconomic status, each additional child in the offspring group resulted in a 2.3% (95% CI 1.9–2.6%, P < 0.001) lower proportional survival of the offspring. Furthermore, we made use of the polygamous population structure and compared offspring of co‐wives in 388 households, thus controlling for variation in resources between compounds. Here, offspring survival decreased 2.8% (95% CI 2.3–4.0%, P < 0.001) for each increase in offspring number. We interpret these data as an apparent quality–quantity trade‐off in human offspring.     

Bigger is better: changes in body size explain a maternal effect of food on offspring disease resistance

Maternal effects triggered by changes in the environment (e.g., nutrition or crowding) can influence the outcome of offspring–parasite interactions, with fitness consequences for the host and parasite. Outside of the classic example of antibody transfer in vertebrates, proximate mechanisms have been little studied, and thus, the adaptive significance of maternal effects on infection is not well resolved. We sought to determine why food‐stressed mothers give birth to offspring that show a low rate of infection when the crustacean Daphnia magna is exposed to an orally infective bacterial pathogen. These more‐resistant offspring are also larger at birth and feed at a lower rate. Thus, reduced disease resistance could result from slow‐feeding offspring ingesting fewer bacterial spores or because their larger size allows for greater immune investment. To distinguish between these theories, we performed an experiment in which we measured body size, feeding rate, and susceptibility, and were able to show that body size is the primary mechanism causing altered susceptibility: Larger Daphnia were less likely to become infected. Contrary to our predictions, there was also a trend that fast‐feeding Daphnia were less likely to become infected. Thus, our results explain how a maternal environmental effect can alter offspring disease resistance (though body size), and highlight the potential complexity of relationship between feeding rate and susceptibility in a host that encounters a parasite whilst feeding.     

Re‐examining extreme longevity of the cave crayfish Orconectes australis using new mark–recapture data: a lesson on the limitations of iterative size‐at‐age models

1. Centenarian species, defined as those taxa with life spans that frequently exceed 100 years, have long been of interest to ecologists because they represent an extreme end point in a continuum of life history strategies. One frequently reported example of a freshwater centenarian is the obligate cave crayfish Orconectes australis, with a maximum longevity reported to exceed 176 years. As a consequence of its reported longevity, O. australis has been used as a textbook example of life history adaptation to the organic‐carbon limitation that characterises many cave‐stream food webs. 2. Despite being widely reported, uncertainties surround the original estimates of longevity for O. australis, which were based on a single study dating from the mid‐1970s. In the present study, we re‐evaluated the growth rate, time‐to‐maturity, female age‐at‐first‐reproduction and longevity of O. australis using a mark–recapture study of more than 5 years based upon more than 3800 free‐ranging individuals from three isolated cave streams in the south‐eastern United States. 3. The results of our study indicate that accurate estimates of the longevity of O. australis are ≤22 years, with only a small proportion of individuals (<5%) exceeding this age. Our estimates for female time‐to‐maturity (4–5 years) and age‐at‐first‐reproduction (5–6 years) are also substantially lower than earlier estimates. 4. These new data indicate that the age thresholds for life history events of O. australis are comparable to other estimates for a modest assemblage of cave and surface species of crayfish for which credible age estimate exists, suggesting that a cave environment per se is not required for the evolution of extreme longevity in crayfish.     

Strongly female‐biased sex allocation in a trivoltine population of Trypoxylon (Trypargilum) opacum Brèthes (Hymenoptera,Crabronidae)

Buschini, M.L.T. and Bergamaschi, A.C.B. 2009. Strongly female‐biased sex allocation in a trivoltine population of Trypoxylon (Trypargilum) opacum Brèthes (Hymenoptera, Crabronidae). —Acta Zoologica (Stockholm) 91 : 433–439. This study was carried in southern Brazil from December 2001 to December 2004. The aim of this paper is to provide additional information on the life‐history and sex allocation of this little known species. This wasp’s species has two alternative life histories: either they pupated immediately and emerged as adults later in the same season (direct development) or they entered diapause, overwintering and pupating the following spring (delayed development). The numerical sex ratio of overwintering and of direct developing wasps were strongly female biased in 2002, 2003 and 2004.     

The Eberth–Katschenko layer in three species of ceratophryines anurans (Anura: Ceratophrydae)

Mangione, S., Garcia, G. and Cardozo, O.M. 2011. The Eberth–Katschenko layer in three species of ceratophryines anurans (Anura: Ceratophrydae). —Acta Zoologica (Stockholm) 92 : 21–26. In this study, we made a morphological analysis by optic microscopy of the dorsal and ventral skin of the body of Ceratophrys cranwellii, Lepidobatrachus llanensis and Chacophrys pierotti to verify if there exist shared integumental characters in relation to the Eberth–Katschenko layer (E–K layer). The presence of cells associated with this layer, historically considered acellular, is one of the leading characters. This study contributes to support that the E–K layer would be the remnant of an ancestral dermal skeleton and not a physiologically significant formation. Hence, the presence of cells associated with the E–K layer would represent a synapomorphy for Ceratophrydae.     

Interaction between Shaoyao–Gancao–Tang and a laxative with respect to alteration of paeoniflorin metabolism by intestinal bacteria in rats

Shaoyao-Gancao-Tang (SGT), a traditional Chinese herbal medicine (Kampo formulation) containing Shaoyao (Paeoniae Radix) and Gancao (Glycyrrhizae Radix), is co-administered with laxative sodium picosulfate as a premedication for relieving the pain accompanying colonoscopy. Paeoniflorin (PF), an active glycoside of SGT, is metabolized into the antispasmodic agent paeonimetabolin-I (PM-I) by intestinal bacteria after oral administration. The objective of the present study was to investigate whether the co-administered laxative (sodium picosulfate) influences the metabolism of PF to PM-I by intestinal bacteria. We found that the PF-metabolizing activity of intestinal bacteria in rat feces was significantly reduced to approximately 34% of initial levels by a single sodium picosulfate pretreatment and took approximately 6 days to recover. Repeated administration of SGT after the sodium picosulfate pretreatment significantly shortened the recovery period to around 2 days. Similar results were also observed for plasma PM-I concentration. Since PM-I has muscle relaxant activity, the present results suggest that repetitive administration of SGT after sodium picosulfate pretreatment might be useful to relieve the pain associated with colonoscopy.     

The trade-off between number and size of offspring in humans and other primates

Life-history theory posits a fundamental trade-off between number and size of offspring that structures the variability in parental investment across and within species. We investigate this 'quantity-quality' trade-off across primates and present evidence that a similar trade-off is also found across natural-fertility human societies. Restating the classic Smith-Fretwell model in terms of allometric scaling of resource supply and offspring investment predicts an inverse scaling relation between birth rate and offspring size and a (-1/4) power scaling between birth rate and body size. We show that these theoretically predicted relationships, in particular the inverse scaling between number and size of offspring, tend to hold across increasingly finer scales of analyses (i.e. from mammals to primates to apes to humans). The advantage of this approach is that the quantity-quality trade-off in humans is placed into a general framework of parental investment that follows directly from first principles of energetic allocation.     

F-box基因拷贝数目变异的机制研究:以12种果蝇为例

拷贝数目变异是一种对表型变异和生物进化具有重要意义的基因组结构变异.以前的研究表明不同物种中F-box基因的拷贝数目差异较大.为了深入探索拷贝数目变异的式样和机制.我们以12个果蝇近缘种为研究对象,分析了F-box基因的系统发育关系、进化式样以及它们在染色体上的位置.结果发现,虽然各个物种中F-box基因的拷贝数目差别不大(42-47个),但是仍然存在着很多引起拷贝数目变异的基因获得和丢失事件.这说明表面上变化不大的拷贝数目在一定程度上掩盖了频繁发生的基因获得和丢失事件.通过比较这些基因在染色体上的位置,发现只有在亲缘关系很近的物种之间才能鉴定出有明显微共线性关系的基因组区段.我们还发现,造成F-box基因拷贝数目增加的主要机制是散在重复和串联重复,而反转录转座和新基因的非编码区起源也是两种值得注意的机制.此外,序列变异导致的外显子边界变化以及外显子丢失是引起拷贝数目减少的两种机制.在12种果蝇的最近共同祖先中,F-box基因的拷贝数目与现存物种基本相似,但是基因的获得和丢失事件使得现存物种中的F-box基因在构成上已经有了明显的差别.对数目变异的式样及其与基因功能的关系的研究表明,拷贝数目变异是F-box基因家族"生与死"的进化在基因组层面的系统反映,并有可能为表型变异提供了原始材料.     

Diversity of age‐specific reproductive rates may result from ageing and optimal resource allocation

This paper reports the results of a dynamic programming model which optimizes resource allocation to growth, reproduction and repair of somatic damage, based on the disposable soma theory of ageing. Here it is shown that different age‐dependent patterns of reproductive rates are products of optimal lifetime strategies of resource partitioning. The array of different reproductive patterns generated by the model includes those in which reproduction begins at the maximum rate at maturity and then declines to the end of life, or increases up to a certain age and then drops. The observed patterns reflect optimal resource allocation shaped by the level of extrinsic mortality. A continuous decline in the reproductive rate from the start of reproduction is associated with high extrinsic mortality, and an early increase in the reproductive rate occurs under low extrinsic mortality. A long‐lived organism shows a low reproductive rate early in life, and short‐lived organisms start reproduction at the maximum rate.     

Effect of photoperiod on the size–temperature relationship in a pentatomid bug, Dolycoris baccarum

(1) The effects of photoperiod and temperature on nymphal growth, development and adult size in a pentatomid bug, Dolycoris baccarum, were examined. (2) The temperature–size relationship was not stable but was highly affected by the photoperiod, and showed a geographical variation. (3) There were variations also in the developmental period and the growth rate. (4) The responses to temperature and photoperiod can explain the seasonal and geographical adaptations of the insect, and thus are considered to be important seasonal adaptations in D. baccarum.