Why do male snakes have longer tails than females?

In most snake species, males have longer tails than females of the same body length. The adaptive significance of this widespread dimorphism has attracted much speculation, but few tests. We took advantage of huge mating aggregations of red-sided gartersnakes (Thamnophis sirtalis parietalis) in southern Manitoba to test two (non-exclusive) hypotheses about the selective forces responsible for this dimorphism. Our data support both hypotheses. First, relative tail length affects the size of the male copulatory organs (hemipenes). Males with longer tails relative to body length have longer hemipenes, presumably because of the additional space available (the hemipenes are housed inside the tail base). Second, relative tail length affects male mating success. Males with partial tail loss (due to predation or misadventure) experienced a threefold reduction in mating success. Among males with intact tails, we detected strong stabilizing selection on relative tail length in one of the two years of our study. Thus, our data support the notion that sex divergence in tail length relative to body length in snakes reflects the action of sexual selection for male mating success.     

Why do honeybees reject certain flowers?

Summary Honeybees often approach flowers of Lotus corniculatus and then fly away without attempting to extract nectar. These rejected flowers contained 41% less nectar than my random sample. The accepted flowers contained 24% more nectar than my random sample. The differences among these three flower-groups were due to differences in the percent of empty flowers in each group rather than the differences in the absolute amount of nectar. Honeybees increased their foraging efficiency by accepting less empty flowers and rejecting more empty flowers than would be expected if they foraged randomly. There are two possible mechanisms for this discrimination-behavior: either the bees are smelling nectar odor or they are smelling bee scent left by previous visitors to the flower. My results are inconsistent with the hypothesis that bees are basing their decision on nectar smell and suggest that they are using bee scent as a means of identifying empty flowers.     

Why are triploid zebrafish all male?

Adult triploid zebrafish Danio rerio has previously been reported to be all male. This phenomenon has only been reported in one other gonochoristic fish species, the rosy bitterling Rhodeus ocellatus, despite the fact that triploidy is induced in numerous species. To investigate the mechanism responsible, we first produced triploid zebrafish and observed gonad development. Histological sections of juvenile triploid gonads showed that primary growth oocytes were able to develop in the juvenile ovary, but no cortical alveolus or more advanced oocytes were found. All adult triploids examined were male (n = 160). Male triploids were able to induce oviposition by diploid females during natural spawning trials, but fertilization rates were low (1.0 ± 3.1%) compared with diploid male siblings (67.4 ± 16.6%). The embryos produced by triploid sires were aneuploid with a mean ploidy of 2.4 ± 0.1n, demonstrating that triploid males produce aneuploid spermatozoa. After confirming that adult triploids are all male, we produced an additional batch of triploid zebrafish and exposed them (and a group of diploid siblings) to 100 ng/L estradiol (E2) from 5 to 28 dpf. The E2 treated triploids and nontreated triploids were all male. The nontreated diploids were also all male, but the E2 treated diploids were 89% female. This demonstrates that triploidy acts downstream of estrogen synthesis in the sex differentiation pathway to induce male development. Based on this and the observations of juvenile gonad development in triploids, we suggest that triploidy inhibits development of oocytes past the primary growth stage, and this causes female to male sex reversal.     

Why have organelles retained genomes?

The observation that chloroplasts and mitochondria have retained relics of eubacterial genomes and a protein-synthesizing machinery has long puzzled biologists. If most genes have been transferred from organelles to the nucleus during evolution, why not all? What selective pressure maintains genomes in organelles? Electron transport through the photosynthetic and respiratory membranes is a powerful - but dangerous - source of energy. Recent evidence suggests that organelle genomes have persisted because structural proteins that maintain redox balance within bioenergetic membranes must be synthesized when and where they are needed, to counteract the potentially deadly side effects of ATP-generating electron transport.     

Why do microorganisms have aquaporins?

Aquaporins are channel proteins that enhance the permeability of cell membranes for water. They have been found in Bacteria, Archaea and Eukaryotes. However, their absence in many microorganisms suggests that aquaporins do not fulfill a broad role such as turgor regulation or osmoadaptation but, instead, fulfill a role that enables microorganisms to have specific lifestyles. The recent discovery that aquaporins enhance cellular tolerance against rapid freezing suggests that they have ecological relevance. We have identified several examples of large-scale freeze-thawing of microbes in nature and we also draw attention to alternative lifestyle-related functions for aquaporins, which will be a focus of future research.     

Why do schistosomes have separate sexes?

Paul Basch postulates that the familiar Schistosoma of humans evolved from hermaphroditic blood flukes of Mesozoic reptiles as those host became warm-blooded. The reproductive superiority that accompanied tendencies to protandry and protogyny in hermaphroditic blood flukes has led to subsequent sexual separation and dimorphism but substantial fragments of the ancestral contrasex genome persist in present-day males, as shown by common tendencies toward hermaphroditism. In present-day females the loss of the male-specific genome is far more complete and in the process of optimizing reproductive efficiency, present-day females have sacrificed many structural elements including locomotory and pharyngeal muscles. These losses have created dependency on the well-muscled male, whose primary functions seem to be compensatory; ie., physical transport of the female from the point of pairing to the point of egg deposition, stimulating growth and development by pumping blood into the female, who unpaired would starve, plus, less importantly, fertilization of the oocytes.     

Does sunlight have a beneficial influence on certain cancers?

Apperly [1941. The relation of solar radiation to cancer mortality in North America. Cancer Research 1, 191-195] first proposed that increased mortality from cancer in the north than in the south of the USA might be due to the south to north decrease in ambient solar radiation. This inverse association between ambient solar radiation and cancer mortality has been subsequently reported for cancers of the colon, breast, ovary and prostate. While the evidence that sunlight might be related to lower incidence or more favourable outcomes from cancer came initially from ecological studies, case-control and cohort studies have now shown a similar association of sun exposure with risks of colon, breast and prostate cancers in individuals, and other studies in individuals have found that serum and dietary vitamin D levels are associated with reduced risks of colorectal cancer and, less certainly, prostate cancer. Studies in individuals have recently also suggested an effect of sun exposure to reduce risk of non-Hodgkin lymphoma and to increase survival after a diagnosis of melanoma. Data on variation in survival from cancer by season of diagnosis suggest that sun exposure may also improve outcome from cancers of the breast, colon and prostate and Hodgkin lymphoma.     

Why do male chimpanzees defend a group range?

Male chimpanzees, Pan troglodytes, cooperate to defend a community range within which resident females range in smaller core areas. There has been debate over exactly what males are defending, whether mates, territory or both. One hypothesis holds that males are defending mates, and that an increase in community range size will lead directly to the acquisition of more females. However, males frequently attack females as well as males at the edge of the community range. We examined 18 years of observational data on the Gombe chimpanzees to determine the behaviour of males during extragroup encounters, and the consequences of changes in community range size on the number of adult females and indirect measures of food availability. Males were always aggressive to males from other communities, and often attacked adult females, especially those that were not sexually receptive, were older, and/or had more than one offspring. The number of females did not increase with range size, but several measures suggested an increase in food availability with range size. These measures include more time spent in large foraging parties, higher encounter rates with resident females, more encounters with sexually receptive females and higher female reproductive rates. These findings suggest that males defend a feeding territory for their resident females and protect them from sexual harassment. Although a large range may eventually attract more females, this is not an immediate consequence of range expansion. Male number was not correlated with community range size.     

Why do females copulate repeatedly with one male?

For most animals, a small number of copulations is sufficient to fertilize all the eggs that a female will lay at any one time. However, in some species a very high frequency of mating occurs, indicating that individuals copulate many more times than are necessary for fertilization. If copulation behaviour carries costs, then the question arises as to how individuals of both sexes benefit from repeated matings with a single partner. For a male, a high frequency of copulation appears to be advantageous in securing or assuring paternity when his sperm is in competition with those of another male. Since copulation is likely to be as costly for females as it is for males, it is necessary to seek adaptive explanations from the female perspective. Attention is now being focused on why females should copulate repeatedly with a single male.     

Why do protein architectures have boltzmann-like statistics?

A theoretical study has shown that the occurrence of various structural elements in stable folds of random copolymers is exponentially dependent on the own energy of the element. A similar occurrence-on-energy dependence is observed in globular proteins¹ from the level of amino acid conformations to the level of overall architectures. Thus, the structural features stabilized by many random sequences are typical of globular proteins while the features rarely observed in proteins are those which are stabilized by only a minor part of the random sequences. © 1995 Wiley-Liss, Inc.     

Why don't we have a schistosomiasis vaccine?

Over the past 30 years there has been a concerted effort to understand host immune responses to schistosomes, with the ultimate aim of producing a vaccine for human use. In this issue, Bergquist and Colley, in summarizing recent meetings in Cairo, provide a detailed appraisal of progress towards that goal. It seems an appropriate time to ask why, with reference to Schistosoma mansoni, the development of a vaccine has proved so difficult. This question is explored here by Alan Wilson and Pat Coulson.     

Why have child pedestrian death rates fallen?

Pedestrian injuries are a leading cause of childhood mortality and disability. Over the past two decades in Britain child pedestrian death rates have fallen despite large increases in traffic volume. In this paper Roberts examines the likely reasons for this decline. He argues that neither prevention programmes nor improvements in medical care are a plausible explanation and that the decline is most likely the result of a substantial reduction in children''s traffic exposure. He believes, however, that restricting children''s traffic exposure exacerbates socioeconomic differentials in childhood mortality and denies children their right to mobility. Roberts is convinced that one answer is for British transport policy to be aimed at providing mobility equitably rather than struggling to meet the ever increasing demands of car travel.     

Why should an aged male baboon ever transfer troops?

Dispersals by subadult or adult male baboons are common. In contrast, the occasional transfer of aged baboons is puzzling, given the physical dangers of transfer and the cognitive demands of mastering the ecological and social rules of a new troop. The present data suggest a possible explanation for such transfers in a study of two troops of olive baboons (Papio anubis) in Kenya. Aged males who remained in the troop in which they had been dominant were subjected to significantly higher rates of approach-avoid interactions by the current high-ranking cohort (i.e., the individuals they had dominated years past) when compared to males who had transferred into the troop in their old age. Thus, transfer in old age offers the advantage of relative anonymity. Of the 14 males who progressed into older adulthood in the same troop in which they were in their prime, seven ultimately transferred to a different troop. The seven who remained and the seven who transferred did not differ in the rate at which they were subjected to approach-avoid interactions. However, those who remained had significantly higher rates of various affiliative behaviors (copulations, consortships, grooming and contact with females, and positive interactions with infants). Thus, amid the disadvantages of an old age spent in the troop in which a male baboon was in his prime, a high degree of social affiliation might constitute a sufficient disincentive against transferring. © 1996 Wiley-Liss, Inc.     

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins'' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.Key words: actin, Arabidopsis, blue light, kinesin, myosin, organelle movement, phototropinOrganelle movement and positioning are pivotal aspects of the intracellular dynamics in most eukaryotes. Although plants are sessile organisms, their organelles are quickly repositioned in response to fluctuating environmental conditions and certain endogenous signals. By and large, plant organelle movements and positioning are dependent on actin filaments, although microtubules play certain accessory roles in organelle dynamics.^1,2 Actin inhibitors effectively retard the movements of mitochondria,^3–6 peroxisomes,^5,7–11 Golgi stacks,^12,13 endoplasmic reticulum (ER),^14,15 and nuclei.^16–18 These organelles are co-aligned and associated with actin filaments.^{5,7,8,10–12,15,18} Recent progress in this field started to reveal the molecular motility system responsible for the organelle transport in plants.¹⁹Chloroplast movement is among the most fascinating models of organelle movement in plants because it is precisely controlled by ambient light conditions.^20,21 Weak light induces chloroplast accumulation response so that chloroplasts can capture photosynthetic light efficiently (Fig. 1A). Strong light induces chloroplast avoidance response to escape from photodamage (Fig. 1B).²² The blue light-induced chloroplast movement is mediated by the blue light receptor phototropin (phot). In some cryptogam plants, the red light-induced chloroplast movement is regulated by a chimeric phytochrome/phototropin photoreceptor neochrome.^23–25 In a model plant Arabidopsis, phot1 and phot2 function redundantly to regulate the accumulation response,²⁶ whereas phot2 alone is essential for the avoidance response.^27,28 Several additional factors regulating chloroplast movement were identified by analyses of Arabidopsis mutants deficient in chloroplast photorelocation.^29–32 In particular, identification of CHUP1 (chloroplast unusual positioning 1) revealed the connection between chloroplasts and actin filaments at the molecular level.²⁹ CHUP1 is a chloroplast outer membrane protein capable of interacting with F-actin, G-actin and profilin in vitro.^29,33,34 The chup1 mutant plants are defective in both the chloroplast movement and chloroplast anchorage to the plasma membrane,^22,29,33 suggesting that CHUP1 plays an important role in linking chloroplasts to the plasma membrane through the actin filaments. However, how chloroplasts move using the actin filaments and whether chloroplast movement utilizes the actin-based motility system similar to other organelle movements remained to be determined.Open in a separate windowFigure 1Schematic distribution patterns of chloroplasts in a palisade cell under different light conditions, weak (A) and strong (B) lights. Shown as a side view of mid-part of the cell and a top view with three different levels (i.e., top, middle and bottom of the cell). The cell was irradiated from the leaf surface shown as arrows. Weak light induces chloroplast accumulation response (A) and strong light induces the avoidance response (B).Here, we review the recent findings pointing to existence of a novel actin-based mechanisms for chloroplast movement and discuss the differences between the mechanism responsible for movement of chloroplasts and other organelles.