Life cycle of the multiarmed sea star Coscinasterias acutispina (Stimpson, 1862) in laboratory culture: sexual and asexual reproductive pathways

The multiarmed sea star Coscinasterias acutispina generally has 7-10 arms and 2-5 madreporites. It is known to be able to reproduce by asexual fission, and we have previously observed that this species also has the ability to reproduce sexually; however, there has been no report until now of spawning in this species. We succeeded in establishing a long-term culture of juveniles produced by artificial fertilization. Twelve months after the completion of metamorphosis, three individuals had six arms of the same length and a madreporite. At this time, fission occurred in two of these individuals, while the remaining individual underwent fission four months later. Each sea star divided into two halves, provided with three arms each. Thereafter, four or five new arms and two or four madreporites were formed anew in each of the six daughter sea-stars, so that by 30 days after the first fission the number of arms and madreporites in each was similar to that in adults. A second fission occurred in four of these six individuals, four or five months after the first fission, and in three of them the plane of division was the same as that of the first fission. The original three individuals eventually proliferated to 12 by undergoing fission. All individuals had fully developed gonads by 1-3 months after the second fission. Some of them eventually spawned under laboratory culture, and the resulting larvae metamorphosed into juveniles. Our observations demonstrate that individuals of C. acutispina possess the potential for both sexual and asexual reproduction.     

Allelic and genotypic diversity in long-term asexual populations of the pea aphid, Acyrthosiphon pisum in comparison with sexual populations

Many aphid species exhibit geographical variation in the mode of reproduction that ranges from cyclical parthenogenesis with a sexual phase to obligate parthenogenesis (asexual reproduction). Theoretical studies predict that organisms reproducing asexually should maintain higher allelic diversity per locus but lower genotypic diversity than organisms reproducing sexually. To corroborate this hypothesis, we evaluated genotypic and allelic diversities in the sexual and asexual populations of the pea aphid, Acyrthosiphon pisum (Harris). Microsatellite analysis revealed that populations in central Japan are asexual, whereas populations in northern Japan are obligatorily sexual. No mixed populations were detected in our study sites. Phylogenetic analysis using microsatellite data and mitochondrial cytochrome oxidase subunit I (COI) gene sequences revealed a long history of asexuality in central Japan and negated the possibility of the recent origin of the asexual populations from the sexual populations. Asexual populations exhibited much lower genotypic diversity but higher allelic richness per locus than did sexual populations. Asexual populations consisted of a few predominant clones that were considerably differentiated from one another. Sexual populations on alfalfa, an exotic plant in Japan, were most closely related to asexual populations associated with Vicia sativa L. The alfalfa-associated sexual populations harboured one COI haplotype that was included in the haplotype clade of the asexual populations. Available evidence suggests that the sexuality of the alfalfa-associated populations has recently been restored through the northward migration and colonization of alfalfa by V. sativa- associated lineages. Therefore, our results support the theoretical predictions and provide a new perspective on the origin of sexual populations.     

Comparative anatomy and histology of developmental and parasitic stages in the life cycle of the lined sea anemone Edwardsiella lineata

The evolution of parasitism is often accompanied by profound changes to the developmental program. However, relatively few studies have directly examined the developmental evolution of parasitic species from free-living ancestors. The lined sea anemone Edwardsiella lineata is a relatively recently evolved parasite for which closely related free-living outgroups are known, including the starlet sea anemone Nematostella vectensis. The larva of E. lineata parasitizes the ctenophore Mnemiopsis leidyi, and, once embedded in its host, the anemone assumes a novel vermiform body plan. That we might begin to understand how the developmental program of this species has been transformed during the evolution of parasitism, we characterized the gross anatomy, histology, and cnidom of the parasitic stage, post-parasitic larval stage, and adult stage of the E. lineata life cycle. The distinct parasitic stage of the life cycle differs from the post-parasitic larva with respect to overall shape, external ciliation, cnida frequency, and tissue architecture. The parasitic stage and planula both contain holotrichs, a type of cnida not previously reported in Edwardsiidae. The internal morphology of the post-parasitic planula is extremely similar to the adult morphology, with a complete set of mesenterial tissue and musculature despite this stage having little external differentiation. Finally, we observed 2 previously undocumented aspects of asexual reproduction in E. lineata: (1) the parasitic stage undergoes transverse fission via physal pinching, the first report of asexual reproduction in a pre-adult stage in the Edwardsiidae; and (2) the juvenile polyp undergoes transverse fission via polarity reversal, the first time this form of fission has been reported in E. lineata.     

大型水母幼体生长的影响因子研究进展

21世纪以来,中国东、黄海,韩国西海岸以及日本海连年发生大型水母暴发现象,对海洋渔业的生产活动以及海洋生态系统带来巨大的影响。水母暴发形成机制非常复杂,解释其发生机理并有效预报是目前急待解决的问题。大型水母的生活史中有明显的世代交替现象,受精卵,浮浪幼虫,螅状体,足囊,横裂体到碟状体的幼体发育阶段属无性世代,幼蜇发育到成蜇阶段属有性世代。在早期生活史中,螅状体的足囊繁殖与横裂生殖是大型水母无性繁殖的重要方式,对其成体的数量形成至关重要。综述了国内外有关温度、盐度、光以及营养条件对大型水母早期发育阶段的影响研究进展,研究表明温度是影响螅状体发育以及足囊繁殖和横裂生殖的最主要的环境因子;盐度、光和营养条件在适温范围内,均对螅状体和横裂生殖有一定的影响,其上下限随水母种类和发育阶段有所变化。展望了大型水母早期幼体研究的发展趋势,如环境因子对不同种类的大型水母幼体生长机理的影响、多个环境因子对幼体的综合作用、动态的环境因子与大型水母幼体之间的关系等。     

Phenotypic plasticity of reproductive effort in a colonial ascidian,Botryllus schlosseri

Phenotypic plasticity is the capability of a genotype to produce different phenotypes in different environments. Previous studies have indicated phenotypic variability in asexual, male, and female reproduction in Botryllus schlosseri, a hermaphroditic, colonial ascidian, but not explicitly tested for genotype by environment interactions that indicate genetic variation in plastic responses. Consequently, clones derived from an estuarine population were deployed at their native site and a warmer, higher productivity site 10 km up-river. Male reproduction was assayed by testis size, female reproduction by the number of eggs produced, and asexual reproduction by colony growth rate. To test for ontogenetic effects, data were collected from two different generations of zooids born in the field. Analyses of variance indicated plasticity in asexual and female reproduction during the first zooid generation and plasticity in all three traits during the third zooid generation. Reaction norms varied significantly among genotypes in direction and magnitude for asexual reproduction at both times, implying that selection on asexual reproduction is weak. Sperm production during the third zooid generation was significantly lower at the nonnative site, but there was no genotype by environment interaction. The reaction norms for female reproduction varied significantly among genotypes in direction and magnitude during the first zooid generation, but only varied in magnitude during the third generation, with egg production being higher in all genotypes at the nonnative site. Comparisons of weighted frequency distributions between sites demonstrated that differences in egg production in the third generation were due to increases in the proportion of reproductive zooids within a colony. The greater emphasis on female reproduction at a site associated with higher food availability and temperature, and the greater emphasis on male reproduction at a colder, food-limited site, supports predictions from sex allocation theory.     

Fission in Convolutriloba longifissura: asexual reproduction in acoelous turbellarians revisited

Studies of the asexual reproduction of Convolutriloba longifissura (Acoela, Acoelomorpha) revealed that there is no longitudinal fission of the whole animal as has been described by Bartolomaeus and Balzer (1997) . Instead, the first step is a transverse fission. This results in the detachment of the caudal fourth of the mother animal. The detached part forms what we call the butterfly stage, which initially has no mouth and no eye fields. This stage gives rise to two new individuals by a longitudinal fission. Within 2–3 days the eye fields and a mouth develop in each of the two progenies formed in this way. In the meantime the mother individual grows and develops the three typical caudal lobes. The mother animal can repeat this process resulting in three individuals every fourth day. The finding of this new pattern of reproduction in the Acoela has prompted us to review the various ways by which asexual reproduction occurs in the group. The peculiar combination of few cases but high diversity of asexual reproduction in the Acoela is discussed from an evolutionary point of view.     

Asexual reproduction and the turbellarian archetype

The turbellarian archetype is widely believed to have been a hermaphrodite lacking asexual reproduction, and such asexual reproduction as is now seen in the Turbellaria (as paratomy and architomy) is generally assumed to have arisen secondarily several times independently. Asexual reproduction clearly prevails among the most primitive metazoans such as the placozoans, sponges, and radiates, however, and if the Platyhelminthes is indeed an early offshoot of bilaterian evolution, as some have claimed, then it is reasonable to expect asexual reproduction to be a primitive feature of the Turbellaria. Asexual reproduction by paratomy or architomy is found in all three main evolutionary lines of the Turbellaria and is most common among primitive groups such as the Catenulida and Macrostomida. The discovery of a new, apparently primitive marine genus of Macrostomida having paratomy widens the known incidence of asexual reproduction within that order. The presence of a muscle ring around the gut of several distantly related genera of the Macrostomida and similarities this ring shows with septa in the division plane of paratomizing species are evidence that paratomy was a feature of the stem species for this order — a feature only secondarily lost in most macrostomids — and suggest that asexual reproduction is a primitive feature for the Platyhelminthes as a whole.     

Switching from asexual to sexual reproduction in the planarian Dugesia ryukyuensis: change of the fissiparous capacity along with the sexualizing process

Asexual worms of fissiparous strain of the planarian Dugesia ryukyuensis switch from asexual to sexual reproduction, if they are fed with sexually mature worms of Bdellocephala brunnea. This suggests that the sexually mature worms have a sexualizing substance(s) that induces the sexuality in the asexual worms. Here, we found by analysis of the sexualization that the cessation of the fission, namely their asexual reproduction, occurs immediately after the acquisition of sexuality. This result suggests that the downstream mechanisms induced by the putative sexualizing substance in B. brunnea become responsible for the cessation of fission. We also found that the decapitation triggers fission in the worms even after the acquisition of sexuality if they are not sexually mature, while the fully sexualized worms never fission even though they are decapitated. This result suggests that the cessation of fission takes place via at least two steps: (1) the mechanisms associated with the cephalic system; (2) other mechanisms independent of cephalic control.     

Observations onCyanotheca longipes Pascher

Summary Morphological and cytological investigations conducted onCyanoiheca longipes Pascher, revealed the presence of a nucleus, and the asexual and sexual reproduction of the organism is described. It was found that the asexual reproduction is either a modified binary fission or by means of formation of numerous endospores. The sexual reproduction consists of pedogamy which takes place in the stalk.These observations excludeCyanotheca from the division of Cyanophyta.Some speculations about the systematic position of this microorganism are discussed without giving a definite answer about its correct place in the plant kingdom.     

Studies of the fissiparous holothurian Holothuria parvula (Selenka) (Echinodermata: Holothuroidea)

A Bermudan population of the fissiparous holothurian Holothuria parvula (Selenka) was sampled over a 13-month period (1984–1985). Fission was most frequent in the summer when water temperatures were > 25 °C. During fission, the holothurian split into roughly equal parts, and there was little difference in survival of the oral and anal ends. Regeneration of a new gut is a priority and feeding was possible within 2 months of fission. The majority of growth following fission occurred between April and July, just prior to the peak occurrence of fission. Many individuals were fully regenerated within a year, so fission is possibly an annual event. Individuals showing evidence of multiple fission were found. The capacity for sexual reproduction was limited and it appeared to occur mainly during the summer, which was also the peak period for asexual reproduction. No small (< 18 mm) individuals were ever found suggesting that larval recruitment to this population had not recently been successful. The population has probably been maintained recently by fission.     

Fine‐scale asexual and sexual reproduction in Quercus crispula var. horikawae,a stunted shrub oak,on a mountain with deep snow in central Japan

Quercus crispula var. horikawae, a stunted shrub oak, occurs on mountains with deep snow on the Sea of Japan side of Japan. This oak generates patches of multiple creeping stems. It is unclear whether these patches are the result of asexual or sexual reproduction, or both. We therefore aimed to describe the clonal structure and gene flow in Q . crispula var. horikawae on Mount Nasu in central Japan by using nuclear microsatellites. Genotypes of 331 stems with no distinct connection with roots and creeping stems above the ground were determined using nine loci in two study plots, and 64 acorns from three mother genets in a plot were determined using eight loci. The results of the clonal identification indicated that the patches consisted of 51 genets; at least 85% of the stems may have been derived from asexual reproduction through sprouting and layering. The prominence of asexual reproduction may be a result of adaptation to the snowy environment. In contrast, 15% of the ramets in the study plots probably originate via sexual reproduction by seedling regeneration. Analyses of the spatial genetic structure and paternity showed that limited ability of the pollen and seeds to disperse might result in the spatial aggregations of closely related offspring at a relatively short distance (<10 m), and inbreeding, a factor that might reduce sexual reproduction, was not observed. Thus, sexual reproduction could be reduced by ecological rather than genetic factors, namely the hindrance of seedling regeneration by the dense coverage of dwarf bamboo (Sasa) on the forest floor.     

Asexual reproduction in Pygospio elegans claparède (Annelida, Polychaeta) in relation to parasitism by Lepocreadium setiferoides (Miller and Northup) (Platyhelminthes, Trematoda)

Life-history theory predicts that parasitized hosts should alter their investment in reproduction in ways that maximize host reproductive success. I examined the timing of asexual reproduction (fragmentation and regeneration) in the polychaete annelid Pygospio elegans experimentally exposed to cercariae of the trematode Lepocreadium setiferoides. Consistent with adaptive host response, polychaetes that became infected by metacercariae of trematodes fragmented sooner than unexposed controls. Parasites were not directly associated with fission in that exposed polychaetes that did not become infected also fragmented earlier than controls. For specimens of P. elegans that were not exposed to trematodes, new fragments that contained original heads were larger than those that contained original tails, whereas original head and tail fragments did not differ in size for infected polychaetes. In infected specimens, metacercariae were equally represented in original head and tail fragments and were more likely to be found in whichever fragment was larger. Despite early reproduction, parasitism was still costly because populations of P. elegans exposed to parasites were smaller than controls when measured 8 weeks later and because exposure to cercariae reduced survivorship of newly divided polychaetes. Taken together, my results suggest that early fragmentation is a host response to minimize costs associated with parasitism.     

The eco-evolutionary importance of reproductive system variation in the macroalgae: Freshwater reds as a case study

The relative frequency of sexual versus asexual reproduction governs the distribution of genetic diversity within and among populations. Most studies on the consequences of reproductive variation focus on the mating system (i.e., selfing vs. outcrossing) of diploid-dominant taxa (e.g., angiosperms), often ignoring asexual reproduction. Although reproductive systems are hypothesized to be correlated with life-cycle types, variation in the relative rates of sexual and asexual reproduction remains poorly characterized across eukaryotes. This is particularly true among the three major lineages of macroalgae (green, brown, and red). The Rhodophyta are particularly interesting, as many taxa have complex haploid–diploid life cycles that influence genetic structure. Though most marine reds have separate sexes, we show that freshwater red macroalgae exhibit patterns of switching between monoicy and dioicy in sister taxa that rival those recently shown in brown macroalgae and in angiosperms. We advocate for the investigation of reproductive system evolution using freshwater reds, as this will expand the life-cycle types for which these data exist, enabling comparative analyses broadly across eukaryotes. Unlike their marine cousins, species in the Batrachospermales have macroscopic gametophytes attached to filamentous, often microscopic sporophytes. While asexual reproduction through monospores may occur in all freshwater reds, the Compsopogonales are thought to be exclusively asexual. Understanding the evolutionary consequences of selfing and asexual reproduction will aid in our understanding of the evolutionary ecology of all algae and of eukaryotic evolution generally.     

THE ORIGIN AND DISTRIBUTION OF CLONAL DIVERSITY IN ALSOPHILA POMETARIA (LEPIDOPTERA: GEOMETRIDAE)

A survey of spatial and temporal variation in the frequency of electrophoretically defined genotypes in the geometrid moth Alsophila pometaria revealed a high diversity of uncommon or rare asexual genotypes and clinal distributions of two of the more common clones. There was substantial year-to-year variation in genotype frequencies in seven of eleven sites. Progeny tests have revealed that sexual reproduction is uncommon in two populations and that new asexual genotypes arise from the sexual population. The recurrent origin of asexual genotypes is likely to account for the high genetic and ecological diversity of the asexual contingent of this species' populations, in contrast to the lower genetic diversity in some obligately asexual species in which such recruitment does not occur.     

Sexual and asexual reproduction of the holothurian Stichopus chloronotus (Echinodermata): a comparison between La Réunion (Indian Ocean) and east Australia (Pacific Ocean)

Summary

Stichopus chloronotus (Brandt, 1835) is one among nine aspidochirotide holothurian species known to reproduce both sexually by broadcast spawning and asexually by transverse fission. New data on the sexual cycle of this species in La Réunion are presented here and information available on sexual and asexual reproduction in this species is summarised. Sexual reproduction on La Réunion shows a distinct seasonality with a main spawning period in the warm season (November-February). The spawning period the Great Barrier Reef appears to be at the same time. Some intriguing deviations from unity in sex-ratio, usually biased towards more male individuals, have been observed in both geographic regions (sex ratio at La Reunion 31:1). New data on the asexual reproduction of this species in La Réunion confirm the high rates of fission. The peak of asexual reproduction in both the Indian and Pacific Ocean was observed in winter (June-July). Thus, asexual reproduction in this species occurs outside the season for sexual reproduction. The rate of asexual reproduction appears to vary between sample locations. However, results of population genetic studies on S. chloronotus (Uthicke et al., 1999; Uthicke et al., 2001) indicated that in most populations investigated a maximum of about 60% of all individuals may be derived from sexual recruitment. Cluster analyses on genetic distances between populations grouped populations within Oceans together, with the exception of one sample from a nearshore reef of the GBR. Although genetic differences between the two regions exist, these are relatively small regarding the large geographic distance. We conclude that asexual reproduction in S. chloronotus is important to maintain local population sizes, but that larval exchange between populations mediated by sexual reproduction is important for colonisation of new areas and to provide connectivity between populations. Here, we present the first synthesis of these phenomena for a holothurian species.     

Reproductive modes in the epixylic hepatic Anastrophyllum hellerianum

The frequency and dynamics of sexual and asexual reproduction were investigated in a dioecious epixylic hepatic, Anastrophyllum hellerianum, which has declined in recent decades in Finland as a consequence of forestry practices. In our investigation asexual reproduction by gemmae was the dominant mode of reproduction and specialised gemmiparous shoots were present in all colonies studied. The proportions of dead shoots were considerably higher among sex-expressing than among non-sexexpressing shoots. Our results suggest that lower reproductive investment is required for asexual than for sexual reproduction. For instance, no trade-off is detected between asexual reproduction and survival of the gemmiparous shoots in A. hellerianum. Sexual reproduction occurred only in 12% of the colonies and it was promoted by the following factors: medium shoot density, high proportion of sex-expressing shoots, an even sex ratio and very short distances between individuals representing opposite sexes. The ratio of dead males to dead females was significantly female-biased, which suggests higher mortality among female shoots. At the level of individual shoots, more spores than gemmae were produced. However, as a consequence of the low frequency of sporophyte-bearing shoots, gemma production highly exceeded spore production at the colony level. Furthermore, cultivation tests of the propagules showed that gemmae germinate faster than spores.     

Intraspecific genetic structure,divergence and high rates of clonality in an amphi‐Atlantic starfish

Intraspecific genetic diversity and divergence have a large influence on the adaption and evolutionary potential of species. The widely distributed starfish, Coscinasterias tenuispina, combines sexual reproduction with asexual reproduction via fission. Here we analyse the phylogeography of this starfish to reveal historical and contemporary processes driving its intraspecific genetic divergence. We further consider whether asexual reproduction is the most important method of propagation throughout the distribution range of this species. Our study included 326 individuals from 16 populations, covering most of the species’ distribution range. A total of 12 nuclear microsatellite loci and sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene were analysed. COI and microsatellites were clustered in two isolated lineages: one found along the southwestern Atlantic and the other along the northeastern Atlantic and Mediterranean Sea. This suggests the existence of two different evolutionary units. Marine barriers along the European coast would be responsible for population clustering: the Almeria–Oran Front that limits the entrance of migrants from the Atlantic to the Mediterranean, and the Siculo‐Tunisian strait that divides the two Mediterranean basins. The presence of identical genotypes was detected in all populations, although two monoclonal populations were found in two sites where annual mean temperatures and minimum values were the lowest. Our results based on microsatellite loci showed that intrapopulation genetic diversity was significantly affected by clonality whereas it had lower effect for the global phylogeography of the species, although still some impact on populations’ genetic divergence could be observed between some populations.     

Radical modification of the A-P axis and the evolution of asexual reproduction in Convolutriloba acoels

Acoel worms in the genus Convolutriloba are remarkable in that closely related, morphologically very similar species reproduce asexually by dramatically different processes. Transverse fission, longitudinal fission, and reversed-polarity budding all occur within this genus, indicating an unparalleled ability to alter the A-P axis. Convolutriloba thus offers an exceptional opportunity to investigate the development and evolution of asexual reproduction. Molecular phylogenetic analysis indicates that reversed-polarity budding is ancestral and fission is derived for the genus. A clear difference between budding and fission is indicated by the development of the nervous system, which forms de novo during budding, but regenerates largely by extensions of remaining components of the nervous system during both types of fission. Despite this and other differences between fission and budding, localized muscle disorganization coupled with behaviorally mediated tearing are characteristic of both transverse fission and reversed-polarity budding (though not longitudinal fission), suggesting that a homologous tissue-separation mechanism underlies these two outwardly quite different asexual reproductive modes. We suggest that the ability to split the posterior axis field into two adjacent fields, manifested during both reversed-polarity budding and longitudinal fission, may have been a driving force behind the diversification of asexual reproductive mode in this group.     

Asexual Reproduction and Segmental Regeneration, but not Morphallaxis, are Inhibited by Boric Acid in Lumbriculus variegatus (Annelida: Clitellata: Lumbriculidae)

Body fragmentation, in some animal groups, is a mechanism for survival and asexual reproduction. Lumbriculus variegatus (Müller, 1774), an aquatic oligochaete worm, is capable of regenerating into complete individuals from small body fragments following injury and reproduces primarily by asexual reproduction. Few studies have determined the cellular mechanisms that underlie fragmentation, either regenerative or asexual. We utilized boric acid treatment, which blocks regeneration of segments in amputated fragments and blocks architomic fission during asexual reproduction, to investigate mechanistic relationships and differences between these two modes of development. Neural morphallaxis, involving changes in sensory fields and giant fiber conduction, was detected in amputated fragments in the absence of segmental regeneration. Furthermore, neural morphallactic changes occurred as a result of developmental mechanisms of asexual reproduction, even when architomy was prevented. These results show that fragmentation in L. variegatus, during injury or asexual reproduction, employs developmental and morphallactic processes that can be mechanistically dissociated by boric acid exposure. In regeneration following injury, compensatory morphallaxis occurred in response to fragmentation. In contrast, anticipatory morphallaxis was induced in preparation for fragmentation during asexual reproduction. Thus, various forms of regeneration in this lumbriculid worm can be activated independently and in different developmental contexts.