Evolution and Ecology of Parthenogenesis in Earthworms

A model for the origin of parthenogenesis in hermaphroditesis developed. If a dominant mutation causing parthenogeneticdevelopment of eggs without affecting meiotic production ofsperm arises, the parthenogens will increase in frequency tofixation. Concomitantly, there is selection for reallocationof resources from male to female-related functions in both parthenogeneticand sexual individuals. Occasional fertilization of unreducedeggs may produce polyploid clones. Both the loss of male-relatedstructures and polyploidy are common in parthenogenetic earthworms.Parthenogenesis should be favored in patchy and temporally unstablehabitats, in which -selection may be expected, because it facilitatescolonization and rapid population growth, and because selectionby the biotic component of the environment presumably is reduced.Parthenogenetic earthworms commonly occur in decaying logs,leaf litter, and the upper, organic layers of the soil, whereassexual species more often inhabit the deeper, more stable soilhorizons. Long-term persistence of clones depends on their abilityto survive and reproduce under a variety of environmental conditions.It is proposed that successful clones possess "general purpose"genotypes that allow persistence in spite of temporal changesand facilitate active dispersal through heterogeneous environmentsbetween patches of prime habitat. Two common clones of the parthenogeneticearthworm Octolasion tyrtaeum seem to possess general purposegenotypes, as they occur in a wide variety of soil and habitattypes and are geographically widespread.     

Sexual Conflict over the Maintenance of Sex: Effects of Sexually Antagonistic Coevolution for Reproductive Isolation of Parthenogenesis

Sexual reproduction involves many costs. Therefore, females acquiring a capacity for parthenogenetic (or asexual) reproduction will gain a reproductive advantage over obligately sexual females. In contrast, for males, any trait coercing parthenogens into sexual reproduction (male coercion) increases their fitness and should be under positive selection because parthenogenesis deprives them of their genetic contribution to future generations. Surprisingly, although such sexual conflict is a possible outcome whenever reproductive isolation is incomplete between parthenogens and the sexual ancestors, it has not been given much attention in the studies of the maintenance of sex. Using two mathematical models, I show here that the evolution of male coercion substantially favours the maintenance of sex even though a female barrier against the coercion can evolve. First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes. This is because the coevolution between the two traits additionally involves sex-ratio selection, that is, an increase in parthenogenetic reproduction leads to a female-biased population sex ratio, which will enhance reproductive success of more coercive males and directly promotes the evolution of the coercion among males. Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens. These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.     

Systematic and Evolutionary Implications of Parthenogenesis in the Hymenoptera

SYNOPSIS. Two types of parthenogenesis, arrhenotoky and thelytoky,exist in the Hymenoptera. Arrhenotoky, the development of malesfrom unfertilized eggs, is present in all wasps and bees. Thelytoky,the development of diploid females from unfertilized eggs, ispresent in a few species. Two types of thelytoky, apomixis andautomixis, are known. Most thelytokous Hymenoptera are automictic.No meiosis, only mitosis, occurs in apomixis. Meiosis does occurin automixis, allowing crossing-over and segregation of genes.Advantages of thelytoky are that heterotic combinations becomefixed, gene loss is reduced, and reproduction requires onlya single individual. One advantage of arrhenotoky is that geneticload in males is eliminated. Both environmental and geneticfactors contribute to sex-determination in the haplodiploidsystem of Hymenoptera. Haplodiploidy can facilitate the evolutionof social behavior. Parthenogenesis creates some taxonomic problemssince thelytokous clones do not fit the generally accepted biologicalspecies concept. Some members of bisexual populations probablyacquirethelytoky, forming their own clones, races, or species.     

Reproductive Plasticity in Freshwater Invader: From Long-Term Sperm Storage to Parthenogenesis

Orconectes limosus, a North American crayfish species, is one of the most important aquatic invaders in European inland waters. Despite more than 120 years occurrence in Europe and intense research, there are still gaps in knowledge of its life history and ecology. Investigation into O. limosus invasive success requires identifying the mechanisms that enabled them to establish dense and widespread populations from small initial numbers without observable limitation by an introduction bottleneck. In part, O. limosus success may lie in its ability to reproduce by facultative parthenogenesis. Moreover, there are possible other mating scenarios, because of two mating seasons (autumn and spring) in O. limosus. This work investigated the effect of four reproductive scenarios (autumn mating only, spring mating only, autumn and spring mating, and without mating) on the reproductive success of O. limosus. Females successfully reproduced in all tested mating regimes using parthenogenesis as well as log term sperm storage. This reproductive plasticity likely facilitates the overwhelming success of O. limosus spread and establishment in new localities. It can explain the spread of O. limosus from the initial introduction of 90 specimens to most of continental Europe and Great Britain. These conclusions imply a serious threat, not only for autochthonous European astacofauna, but for other aquatic organisms as well as entire ecosystems.     

山地麻蜥繁殖生态的研究

1997年 3月到 1 998年 8月 ,在徐州市近郊的牛头山对山地麻蜥的繁殖生态进行了研究。 3～ 7月为繁殖期 ,其中 4～ 5月为交配产卵盛期。雌雄性比为 1∶1 6。从交配到产卵为 2 6天。卵白色、革质、椭圆形。长径平均为 1 3 7( 1 2 3～ 1 53 )cm ,短径平均为 0 86( 0 7～ 1 0 6)cm ,卵重平均为 0 6( 0 4～0 85) g。每窝平均产卵 4 1 ( 3～ 5)枚。孵化期为 50天。初孵出的小麻蜥尾端为蓝绿色 ,有金属光泽 ,1个月后逐步消失。     

植物生殖生态学研究进展

植物生殖生态学研究进展苏智先张素兰（四川师范学院生物多样性研究中心,南充６３７００２）钟章成（西南师范大学生物地理研究所,重庆６３０７１５）ＡｄｖａｎｃｅｓｉｎＰｌａｎｔＲｅｐｒｏｄｕｃｔｉｖｅＥｃｏｌｏｇｙ．ＳｕＺｈｉｘｉａｎ,ＺｈａｎｇＳｕｌａｎ...     

Human Ovarian Function and Reproductive Ecology: New Hypotheses

A series of eight hypotheses is presented, based on the results of current research, concerning the responsiveness of the human ovary to constitutional and environmental variables. These hypotheses are motivated by a theoretical position that seeks to understand human reproductive physiology as the product of natural selection. The hypotheses are: (1) Ovarian responsiveness occurs along a graded continuum. (2) The graded continuum of response forms a final common pathway for various "stresses." (3) Ovarian function tracks energy balance, not simply nutritional status. (4) Ovarian function tracks aerobic activity independently of energy balance. (5) Additive interactions characterize the interaction of constitutional and environmental factors modulating ovarian function. (6) Reproductive maturation is synchronized with skeletal maturation, especially of the pelvis. (7) Peak ovarian function is not ordinarily achieved until the early twenties. (8) Late reproductive maturation is associated with a slower rise in indices of ovarian function with age, and a lower level of ovarian function in adulthood. Together, these hypotheses provide for two, non-exclusive theories of facultative modulation of female reproductive effort. One theory views ovarian function as responsive to the prospects for positive reproductive outcome as these may be affected by maternal age, maturation, energy balance, and activity level. The second theory views ovarian function as responsive in a similar way to the need to maintain long-term maternal energy balance.     

进化生态学的产生与发展

进化论、遗传学与生态学是在不同时期发展起来的独立学科,然而,随着科学的发展,们越来越认识到它们之间存在着密切的关系。Petrusewicz(1959)(见Shvarts,1977)曾写过《达尔文进化论是一种生态理论》一书,强调进化在本质上是一生态过程。Hutchinson(1965)写了一部题名为《生态学舞台和进化节目》一书,认为生态的布景可以作为进化过程的舞台。但是,最早提出进化生态学这一概念的则是Orians(1962)。进化生态学从产生到现在虽然只有20多年的历史,但已成为生态学研究的重要领域。下面从几个方面谈谈进化生态学的产生与发展。     

贵州疣螈繁殖生态

贵州疣螈(Tylototriton kweichowensis)为中国特有种,国家Ⅱ级重点保护野生动物,对生境变化具有重要指示意义。2018年9月至2019年10月,在贵州省毕节市撒拉溪石漠化综合治理示范区,对贵州疣螈栖息地、形态特征、繁殖行为进行了野外观测。调查显示,贵州疣螈栖息于山塘、山泉、蓄水池、临时性积水坑(塘)等水域,个体全长、尾长和体重雌螈均显著高于雄螈。贵州疣螈于4月18日雷雨天气后破眠外出活动,繁殖始于4月29日,最晚见于8月8日,高峰期为5至6月;繁殖期贵州疣螈的性比总体上偏雄,但在产卵期性比偏雌;雌、雄螈抱对时间几分钟到40min不等;抱对结束后开始排精、纳精;产卵活动在纳精后的1～2 d进行,卵产于繁殖场水底、草或石头上;卵的孵化率为55%,平均孵化期8d,幼体完成变态发育需130d。研究表明,贵州疣螈的繁殖与发育受降雨、水量、温度变化的影响较大,且繁殖场所相对较为固定,容易受到人类活动的干扰。因此,在石漠化治理和生态修复时注重贵州疣螈的栖息地保护,必要时应人工新建稳固繁殖场,保障其生态繁衍。     

Generalization in Ecology and Evolutionary Biology: From Hypothesis to Paradigm

We argue that broad, simplegeneralizations, not specifically linked tocontingencies, will rarely approach truth in ecologyand evolutionary biology. This is because mostinteresting phenomena have multiple, interactingcauses. Instead of looking for single universaltheories to explain the great diversity of naturalsystems, we suggest that it would be profitable todevelop general explanatory frameworks. A frameworkshould clearly specify focal levels. The process orpattern that we wish to study defines our level offocus. The set of potential and actual states at thefocal level interacts with conditions at thecontiguous lower and upper levels of organization,through sets of many-to-one and one-to-manyconnections. The number of initiating conditions andtheir permutations at the lower level define thepotential states at the focal level, whereas theactual state is constrained by the upper-levelboundary conditions. The most useful generalizationsare explanatory frameworks, which are road maps tosolutions, rather than solutions themselves. Suchframeworks outline what is understood about boundaryconditions and initiating conditions so that aninvestigator can pick and choose what is required toeffectively understand a specific event or situation. We discuss these relationships in terms of examplesinvolving sex ratio and mating behavior, competitivehierarchies, insect life-histories and the evolutionof sex.     

Sex in an Evolutionary Perspective: Just Another Reaction Norm

It is common to refer to all sorts of clear-cut differences between the sexes as something that is biologically almost inevitable. Although this does not reflect the status of evolutionary theory on sex determination and sexual dimorphism, it is probably a common view among evolutionary biologists as well, because of the impact of sexual selection theory. To get away from thinking about biological sex and traits associated with a particular sex as something static, it should be recognized that in an evolutionary perspective sex can be viewed as a reaction norm, with sex attributes being phenotypically plastic. Sex determination itself is fundamentally plastic, even when it is termed “genetic”. The phenotypic expression of traits that are statistically associated with a particular sex always has a plastic component. This plasticity allows for much more variation in the expression of traits according to sex and more overlap between the sexes than is typically acknowledged. Here we review the variation and frequency of evolutionary changes in sex, sex determination and sex roles and conclude that sex in an evolutionary time-frame is extremely variable. We draw on recent findings in sex determination mechanisms, empirical findings of morphology and behaviour as well as genetic and developmental models to explore the concept of sex as a reaction norm. From this point of view, sexual differences are not expected to generally fall into neat, discrete, pre-determined classes. It is important to acknowledge this variability in order to increase objectivity in evolutionary research.