Sex allocation predicts mating rate in a simultaneous hermaphrodite

Sexual selection theory for separate-sexed animals predicts that the sexes differ in the benefit they can obtain from multiple mating. Conventional sex roles assume that the relationship between the number of mates and the fitness of an individual is steeper in males compared with females. Under these conditions, males are expected to be more eager to mate, whereas females are expected to be choosier. Here we hypothesize that the sex allocation, i.e. the reproductive investment devoted to the male versus female function, can be an important predictor of the mating strategy in simultaneous hermaphrodites. We argue that within-species variation in sex allocation can cause differences in the proportional fitness gain derived through each sex function. Individuals should therefore adjust their mating strategy in a way that is more beneficial to the sex function that is relatively more pronounced. To test this, we experimentally manipulated the sex allocation in a simultaneously hermaphroditic flatworm and investigated whether this affects the mating behaviour. The results demonstrate that individuals with a more male-biased sex allocation (i.e. relatively large testes and small ovaries) are more eager to mate compared with individuals with a more female-biased sex allocation (i.e. relatively small testes and large ovaries). We argue that this pattern is comparable to conventional gender roles in separate-sexed organisms.     

Experimentally evolved and phenotypically plastic responses to enforced monogamy in a hermaphroditic flatworm

Sexual selection is considered a potent evolutionary force in all sexually reproducing organisms, but direct tests in terms of experimental evolution of sexual traits are still lacking for simultaneously hermaphroditic animals. Here, we tested how evolution under enforced monogamy affected a suite of reproductive traits (including testis area, sex allocation, genital morphology, sperm morphology and mating behaviour) in the outcrossing hermaphroditic flatworm Macrostomum lignano, using an assay that also allowed the assessment of phenotypically plastic responses to group size. The experiment comprised 32 independent selection lines that evolved under either monogamy or polygamy for 20 generations. While we did not observe an evolutionary shift in sex allocation, we detected effects of the selection regime for two male morphological traits. Specifically, worms evolving under enforced monogamy had a distinct shape of the male copulatory organ and produced sperm with shorter appendages. Many traits that did not evolve under enforced monogamy showed phenotypic plasticity in response to group size. Notably, individuals that grew up in larger groups had a more male‐biased sex allocation and produced slightly longer sperm than individuals raised in pairs. We conclude that, in this flatworm, enforced monogamy induced moderate evolutionary but substantial phenotypically plastic responses.     

Growth,maturation and survival in a laboratory-reared Rhabdocoelid Turbellarian,Macrostomum orthostylum (BRAUN 1885)

Growth, maturation and survival of a free living turbellarian Macrostomum orthostylum (BRAUN), from a brackish water fish-farm, were studied in the laboratory under a constant temperature of 24 °C. The worms tolerated a wide range of salinity (1 to 30‰). Maximum growth (total length) of 1000 μm was attained in 56 days with a mean growth rate of 15.7 μm d^-1. Minimum maturation time (7 days) and highest longevity (112 days) were recorded in 9%. salinity. Survival period was considerably longer at lower salinities (1 to 10‰) and showed negative relationship with higher salinities (11 to 30‰).     

Regeneration: Rewarding,but potentially risky

Some bilaterally symmetric animals, such as flatworms, annelids, and nemerteans, are renowned for their outstanding regeneration capacity—even a fraction of the body can give rise to a complete new animal. However, not all species of these taxa can regenerate equally well—some cannot regenerate at all. If regeneration was purely beneficial, why cannot all of members of the flat, round, and ribbon worms regenerate? At that, why cannot all other bilaterians, including humans, regenerate as well? Regeneration capacity is an obvious advantage in accidental, predatory, and parasitic loss of body parts and is also closely intertwined with asexual reproduction strategies. Regeneration is suspected to play a role in life span extension or even rejuvenation. An answer for reduced or missing regeneration capacity in many species may be found in limitations of the body plan, high costs, and inherent dangers of regeneration. Defects in adults and juveniles are shown, and similarities between development and regeneration are pointed out. With a focus on some worms, but also highlighting comparisons with other animal taxa, putative reasons for a limited and an advanced regeneration capacity are discussed in this article. Birth Defects Research (Part C) 84:257–264, 2008. © 2008 Wiley‐Liss, Inc.     

Regeneration in <Emphasis Type="Italic">Macrostomum lignano</Emphasis> (Platyhelminthes): cellular dynamics in the neoblast stem cell system

Neoblasts are potentially totipotent stem cells and the only proliferating cells in adult Platyhelminthes. We have examined the cellular dynamics of neoblasts during the posterior regeneration of Macrostomum lignano. Double-labeling of neoblasts with bromodeoxyuridine and the anti-phospho histone H3 mitosis marker has revealed a complex cellular response in the first 48 h after amputation; this response is different from that known to occur during regeneration in triclad platyhelminths and in starvation/feeding experiments in M. lignano. Mitotic activity is reduced during the first 8 h of regeneration but, at 48 h after amputation, reaches almost twice the value of control animals. The total number of S-phase cells significantly increases after 1 day of regeneration. A subpopulation of fast-cycling neoblasts surprisingly shows the same dynamics during regeneration as those in control animals. Wound healing and regeneration are accompanied by the formation of a distinct blastema. These results present new insights, at the cellular level, into the early regeneration of rhabditophoran Platyhelminthes. This work was supported by FWF Grant (P16618; P.I. Rieger, Innsbruck).     

Resource-dependent sex-allocation in a simultaneous hermaphrodite

Most sex allocation theory is based on the relationship between the resource investment into male and female reproduction and the consequent fitness returns (often called fitness-gain curves). Here we investigate the effects of resource availability on the sex allocation of a simultaneously hermaphroditic animal, the free-living flatworm Macrostomum lignano. We kept the worms under different resource levels and determined the size of their testes and ovaries over a period of time. At higher resource levels, worms allocated relatively more into the female function, suggesting a saturating male fitness-gain curve for this species. A large part of the observed effect was due to a correlated increase in body size, showing size-dependent sex allocation in M. lignano. However, a significant part of the overall effect was independent of body size, and therefore likely due to the differences in resource availability. Moreover, in accordance with a saturating male fitness-gain curve, the worms developed the male gonads first. As the group size was kept constant, our results contrast with expectations from sex allocation models that deal with local mate competition alone, and with previous experiments that test these models.     

管大口涡虫生物学特性的观察

报道了分布于安徽、广东和福建一带的大口涡虫属管大口涡虫(Macrostomum tubo)的生物学特性。该涡虫生活于淡水池塘、小河的水生植物的叶子反面。在实验室观察了该涡虫的生活习性。介绍了常年饲养和繁殖管大口涡虫的方法。通过不同方向的连续切片,描述了该涡虫的形态结构。研究结果提示：大口涡虫是涡虫纲教学和动物进化研究的理想实验材料。     

Occurrence,costs and heritability of delayed selfing in a free‐living flatworm

Evolutionary theory predicts that in the absence of outcrossing opportunities, simultaneously hermaphroditic organisms should eventually switch to self‐fertilization as a form of reproductive assurance. Here, we report the existence of facultative self‐fertilization in the free‐living flatworm Macrostomum hystrix, a species in which outcrossing occurs via hypodermic insemination of sperm into the parenchyma of the mating partner. First, we show that isolated individuals significantly delay the onset of reproduction compared with individuals with outcrossing opportunities (‘delayed selfing’) as predicted by theory. Second, consistent with the idea of M. hystrix being a preferential outcrosser under natural conditions, we report likely costs of selfing manifested via reduced hatchling production and offspring survival. Third, we demonstrate that selfing propensity has a genetic basis in this species, with a heritability estimated at 0.43 ± 0.11. Variation in selfing propensity could arise due to differing costs of inbreeding among families; despite marked inter‐family variation in apparent costs of inbreeding, we found no evidence for such a link. Alternatively, selfing propensity might differ across families because of heritable variation in reproductive traits that determine the likelihood of selfing. We speculate that adaptations to hypodermic insemination under outcrossing, most notably a highly modified copulatory stylet (male copulatory organ) and reduced sperm complexity, could also facilitate facultative selfing in this species.     

中国大口涡虫属一新种记述(大口虫目,大口虫科)

记述大口涡虫属1新种,即厦门大口涡虫Macrostomum xiamensis sp.nov..新种的主要鉴别特征是几丁质交接刺光滑、较细长,表面无几丁质瓣膜.其远端呈螺旋状弯曲,弯曲部的平面上夹角达60度,压片后的夹角达100度,螺旋部超过半圈,交接刺末端呈注射器针的末端,开口于螺旋状弯曲面的外侧.标本采自厦门市区菩陀寺的淡水荷花池塘内,水质属富营养型.所有标本保存在深圳大学生命科学学院实验室.     

双雄性交配器官大口涡虫的首次发现

大口涡虫属所有物种均为雌雄同体,具一套交配器官。作者于2015年在广东省的两处淡水环境,首次发现2个具有双雄性交配器官的大口涡虫(Macrostomum sp.)标本。通过对活体、整装片、连续组织切片的显微镜观测,发现2只标本的两套交配器官呈左右排列;每套交配器官具备完整的假储精囊、储精囊、颗粒囊与交配刺;假储精囊与储精囊内具有精子;雄孔分别为1个与2个;交配刺的结构与中国已经记录的物种都不相同。本研究对其做了较为详细的描述,并初步探讨了大口涡虫多交配器官发生的原因。