Size-monomorphic Species Research Articles

Early-life food stress hits females harder than males in insects: A meta-analysis of sex differences in environmental sensitivity.

Fitness consequences of early-life environmental conditions are often sex-specific, but corresponding evidence for invertebrates remains inconclusive. Here, we use meta-analysis to evaluate sex-specific sensitivity to larval nutritional conditions in insects. Using literature-derived data for 85 species with broad phylogenetic and ecological coverage, we show that females are generally more sensitive to food stress than males. Stressful nutritional conditions during larval development typically lead to female-biased mortality and thus increasingly male-biased sex ratios of emerging adults. We further demonstrate that the general trend of higher sensitivity to food stress in females can primarily be attributed to their typically larger body size in insects and hence higher energy needs during development. By contrast, there is no consistent evidence of sex-biased sensitivity in sexually size-monomorphic species. Drawing conclusions regarding sex-biased sensitivity in species with male-biased size dimorphism remains to wait for the accumulation of relevant data. Our results suggest that environmental conditions leading to elevated juvenile mortality may potentially affect the performance of insect populations further by reducing the proportion of females among individuals reaching reproductive age. Accounting for sex-biased mortality is therefore essential to understanding the dynamics and demography of insect populations, not least importantly in the context of ongoing insect declines.

Sex ratio of Western Bluebirds Sialia mexicana is mediated by phenology and clutch size.

Mothers may produce more of one sex to maximize their fitness if there are differences in the cost of producing each sex or there are differences in their relative reproductive value. Breeding date and clutch size are known to influence offspring sex ratios in birds through sex differences in dispersal, social behaviours, differential mortality, and available food resources. We tested if breeding date, clutch size and drought conditions influenced offspring sex ratios in a sexually size-monomorphic species, the Western Bluebird, by interrogating a 21-year dataset. After controlling for differential mortality, we found that hatch dates late in the breeding season were associated with the production of more females, suggesting that the value of producing males declines as the breeding season progresses. When clutch size was taken into account, small clutches yielded significantly more females late in the breeding season compared to the early and middle parts of the breeding season that produced significantly more males. Large clutches early in the season tended to produce more females, although this was not significant. Drought severity was not correlated with sex ratio adjustment. We propose and discuss several explanations for these patterns, including male offspring, but not female offspring, acting as helpers, increased female nestling provisioning late in the breeding season, differences in food abundance, and egg-laying order. Future work will help to uncover the mechanisms leading to these patterns. Identifying patterns and mechanisms of sex ratio skew from long-term datasets is important for informing predictions regarding life-history trade-offs in wildlife populations.

Morphology of testate amoeba Difflugia australis (Playfair, 1918) Gautier-Lièvre et Thomas, 1958 from a subtropical reservoir (southeast China)

Difflugia australis, first described by Playfair (1918), has a unique morphotype. However, in the absence of morphometric data, it has not yet been reliably classified within the largest testate amoeba genus Difflugia. In this study D. australis collected from a subtropical reservoir in southeast China was investigated by means of light and scanning electron microscopy. Basing on biometrical data, we provide an improved diagnosis of this little known species. Difflugia australis is different from other similar congeners (i.e., D. bacillariarum Perty, 1849 and D. elegans Penard, 1890) mainly by the combination of the following features: the shell is broadly ovate, with rounded dome and convex sides converging down to a very short distance from the aperture and diverging suddenly into a short rim (collar). It is usually more or less asymmetrical, with one side being more dilated than the other. The shell surface is slightly smooth, composed of flat siliceous plates of irregular shape and size, mixed with fine grains; microbial spores of comparable forms are spread on the shell surface; particles are often interspersed with a network of organic cement with unique mesh pattern; one (sometimes two) slanting spine-like posterior end of the shell is variable in form; collar is mainly formed by small plates of equal size. The dimensions of the shell are: total shell length 88–106 µm; shell width 53–88 µm; aperture diameter 19–28 µm; collar height 3–6 µm; spine length 3‒23 μm. The size frequency distributions of both total shell length and shell width indicate that it is a size-monomorphic species with low variability.

Morphometric analysis of six natural populations of Difflugia tuberspinifera

The morphometrical variability of six natural populations of Difflugia tuberspinifera was investigated using the statistic methods based on 374 samples from Yangtze River and Pearl River valleys. The size frequency distribution analysis indicated that D. tuberspinifera is a size-monomorphic species with normal distribution of shell height, shell diameter and aperture diameter. The size of spine length, collar height, foreside length and number of conical spines are the most variable. The correlation analysis showed that most characters are inter-correlated with P<0.05. The shell height, shell diameter, aperture diameter, spine length, collar height, rear end length, foreside length, number of aperture tooth-like structures and number of conical spines differed significantly between different populations (P<0.0001), but principal component analysis (PCA) did not clearly distinguish the six populations based on the morphometric data. However, the cluster analysis separated the six populations into two groups, the Pearl River-Yangtze River group and the Yangtze River group. Further, the populations with different shell characters can coexist within the same river valley. Thus, there is a high morphological variability or diversity within the populations of D. tuberspinifera which is related with local environmental conditions.

Males are sensitive — sex-dependent effect of rearing conditions on nestling growth

The sex-dependent effect of environmental conditions on nestlings has been extensively studied in size dimorphic birds. Whether males or females are more sensitive to poor conditions is not yet clear; however, the degree of sexual size-dimorphism, brood size and their interactions seem to influence the pattern. Much less is known about sex-dependent environmental sensitivity in size-monomorphic species, even though it may result in biased sex allocation. We altered the rearing conditions by brood size manipulation in the size-monomorphic collared flycatcher and then examined the sex-specific development of the nestlings. In all analyses, we controlled for the effect of paternity, because one may expect extra-pair young to be of better genetic quality and perform better at least under poor conditions. However, this was not the case, because we did not find any difference in growth rate or fledging size between extra- and within-pair young. We found that male nestlings had the potential for faster growth under favourable conditions, but suffered more under poor conditions. We found no sex × environment interaction for fledging size probably because the growth curves level off before fledging, and the disadvantaged nestlings can catch up with their siblings. The larger sensitivity of males does not explain the previously found seasonal shift in brood sex ratios and contradicts previous findings in another size-monomorphic species where females were more sensitive. This suggests that even in size-monomorphic species, no general rule exists, which determines the more sensitive sex.

Morphology, Biometry, and Taxonomy of Freshwater and Marine Interstitial Cyphoderia (Cercozoa: Euglyphida)

Good taxonomy is essential for ecological, biogeographical, and evolutionary studies of any group of organisms. Therefore, we performed detailed light- and scanning electron microscopy investigations on the shell ultrastructure and biometric analyses of the morphometric variability of five freshwater and marine interstitial testate amoebae of the genus Cyphoderia (C. trochus var. amphoralis, C. ampulla, C. margaritacea var. major, C. compressa, and C. littoralis), isolated from different populations in Bulgaria and Switzerland. Our aims were (1) to clarify the morphological characteristics of these taxa, and (2) to compare the morphology of a given taxon (C. ampulla) among different locations in Bulgaria and Switzerland as a first step towards an assessment of the geographical variation within a supposedly cosmopolitan taxon. Four of the studied taxa are characterized by a well-expressed main-size class and by a small size range of all the characters and can be defined as size-monomorphic species. Based on these results, the following systematic changes are proposed: C. major (Penard, 1891) n. comb. (Syn.: C. margaritacea var. major (Penard, 1891) and C. amphoralis (Wailes & Penard, 1911) n. comb. (Syn.: C. trochus var. amphoralis (Wailes & Penard, 1911)). However, we also show significant morphological variability between the Swiss and Bulgarian populations of C. ampulla, suggesting the possible existence of more than one taxon within this species. Further studies are required to assess (1) if these two morphologically different taxa represent individual species, (2) if so, if more species exist, and if this diversity is due to limited distribution ranges (endemism) or if several closely related taxa occur together in different geographical areas.

Pentagonia zhangduensis nov. spec. (Lobosea, Arcellinida), a new freshwater species from China

The morphology of a new testate amoeba Pentagonia zhangduensis nov. spec. was investigated using light and scanning electron microscopy. The new species was discovered in the sediments of Lake Zhangdu, Hubei Province, China. The low coefficients of variation and normal size frequency distribution suggest that P. zhangduensis is a size-monomorphic species. P. zhangduensis differs from the one other species in this genus ( P. maroccana), by its larger size and its quadrangular cross section with two parallel longitudinal ridges on each of the two flattened sides of the test.

HAVE MALE AND FEMALE GENITALIA COEVOLVED? A PHYLOGENETIC ANALYSIS OF GENITALIC MORPHOLOGY AND SEXUAL SIZE DIMORPHISM IN WEB-BUILDING SPIDERS (ARANEAE: ARANEOIDEA)

Sexual size dimorphism (SSD) can strongly influence the evolution of reproductive strategies and life history. If SSD is extreme, and other characters (e.g., genitalic size) also increase with size, then functional conflicts may arise between the sexes. Spiders offer an excellent opportunity to investigate this issue because of their wide range of SSD. By using modern phylogenetic methods with 16 species of orb-weaving spiders, we provide strong evidence for the "positive genitalic divergence" model, implying that sexual genitalic dimorphism (SGD) increases as SSD increases. This pattern is supported by an evolutionary mismatch between the absolute sizes of male and female genitalia across species. Indeed, our findings reveal a dramatic reversal from male genitalia that are up to 87x larger than female genitalia in size-monomorphic species to female genitalia that are up to 2.8x larger in extremely size-dimorphic species. We infer that divergence in SGD could limit SSD both in spiders, and potentially in other taxa as well. Further, male and female body size, as well as male and female genitalia size, are decoupled evolutionarily. Finally, we show a negative scaling (hypoallometry) of male and female genitalic morphology within sexes. Evolutionary forces specific to each sex, such as larger female size (increased fecundity) or smaller male size (enhanced mate-searching ability), may be balanced by stabilizing selection on relative genitalic size.

Difflugia mulanensis nov. spec., a freshwater testate amoeba from Lake Mulan, China

Testate amoebae are an important and abundant component of aquatic ecosystems. Our knowledge of their distribution and the species inventory in Asia is still limited. This paper describes a new planktonic Difflugia from Lake Mulan, province of Hubei, China. These findings are of taxonomic, as well as biogeographic significance because Difflugia mulanensis nov. spec. is a rare and perhaps the only locally distributed member of the well-known urceolate division within this most species-rich genus of lobose testate amoebae. We have studied its morphology and biometry with light and scanning electron microscopical methods. D. mulanensis differs from related species by its very large wide funnel-shaped apertural collar which normally exceeds the width of the test, a relatively long neck, its rounded apical end and the presence of only one nucleus. All shell parameters are normally distributed with low variability. Only the collar height is more variable. This shows that D. mulanensis is a size-monomorphic species.

HAVE MALE AND FEMALE GENITALIA COEVOLVED? A PHYLOGENETIC ANALYSIS OF GENITALIC MORPHOLOGY AND SEXUAL SIZE DIMORPHISM IN WEB-BUILDING SPIDERS (ARANEAE: ARANEOIDEA)

Sexual size dimorphism (SSD) can strongly influence the evolution of reproductive strategies and life history. If SSD is extreme, and other characters (e.g., genitalic size) also increase with size, then functional conflicts may arise between the sexes. Spiders offer an excellent opportunity to investigate this issue because of their wide range of SSD. By using modern phylogenetic methods with 16 species of orb-weaving spiders, we provide strong evidence for the "positive genitalic divergence" model, implying that sexual genitalic dimorphism (SGD) increases as SSD increases. This pattern is supported by an evolutionary mismatch between the absolute sizes of male and female genitalia across species. Indeed, our findings reveal a dramatic reversal from male genitalia that are up to 87x larger than female genitalia in size-monomorphic species to female genitalia that are up to 2.8x larger in extremely size-dimorphic species. We infer that divergence in SGD could limit SSD both in spiders, and potentially in other taxa as well. Further, male and female body size, as well as male and female genitalia size, are decoupled evolutionarily. Finally, we show a negative scaling (hypoallometry) of male and female genitalic morphology within sexes. Evolutionary forces specific to each sex, such as larger female size (increased fecundity) or smaller male size (enhanced mate-searching ability), may be balanced by stabilizing selection on relative genitalic size.

GENDER AND ENVIRONMENTAL SENSITIVITY IN NESTLING COLLARED FLYCATCHERS

In many vertebrates, males are apparently more affected by adverse environmental conditions, particularly during early stages of development, than are females. Three explanations have been proposed for this pattern. First, sexual size dimorphism (SSD) may result in sexes having different nutritional requirements to achieve the same viability, and males are more commonly the larger sex. Second, reduced performance of males could result from possession of an unguarded sex chromosome combined with environmental dependence in expression of deleterious recessives. Third, the obsesrved difference may be a consequence of possession of a male phenotype, for example, due to higher circulating levels of androgens associated with development of male reproductive organs acting antagonistically on other systems. We used experimental manipulations of rearing conditions, coupled with a molecular genetic technique for gender identification, to test these hypotheses in a population of Collared Flycatchers, Ficedula albicollis, a migrant, hole-nesting passerine bird. Nestlings of that species exhibit sexual size monomorphism, and as with other birds, females are heterogametic. As a consequence, the three hypotheses make different predictions about the way in which gender and the environment will interact. Comparisons of brothers and sisters in a split-brood, partial cross-fostering design revealed no evidence of gender × environment interaction on body size, wing length, body mass, or recruitment to the breeding population in this size monomorphic species. Our results therefore support the first hypothesis, namely, that sex differences in performance in sexually dimorphic species are most likely to be caused by different nutritional requirements. Our experiments allow us to investigate the existence of sex-specific fitness differences across an environmental gradient; such data are important for generating and testing hypotheses relating to adaptive sex allocation. The absence of gender × environment interactions demonstrated here supports recent studies of this species indicating a lack of sex ratio adjustment in response to a related, natural, environmental gradient. The possibility of gender × environment interactions along environmental gradients other than those investigated here should be addressed experimentally.

When Should Avian Parents Differentially Provision Sons and Daughters?

Given the scarcity of data on sex differences in the behavior, provisioning, growth, and development of young birds, this paper is necessarily speculative. Definitive treatments of the topic will have to wait until more empirical data are available. However, at this point, it is clear that factors besides relative body size can affect the food requirements and provisioning rates of young male and female birds. Relationships between food intake and growth rates can be affected by a number of factors, and in species with sexual size dimorphism, these functional relationships are probably different for young males and females. Several hypotheses predict relatively egalitarian feeding of both sexes in sexually size-dimorphic species or preferential feeding of the members of one sex in sexually size-monomorphic species. These include hypotheses related to competition among the siblings for parental provisioning, competition between young around the time of independence, and sex-biased dispersal in the period immediately following independence. In addition, there are suggestions that male and female young of the same age might allocate food to different components of growth and development, as a result of different developmental schedules or of selection for the early maturation of structures that enhance competitive ability. Hence, the topic of sex-biased provisioning in birds would seem to be a fertile field for further discussion and empirical studies.