Trematode life cycles: short is sweet?

Complex life cycles are a hallmark of parasitic trematodes. In several trematode taxa, however, the life cycle is truncated: fewer hosts are used than in a typical three-host cycle, with fewer transmission events. Eliminating one host from the life cycle can be achieved in at least three different ways. Some trematodes show even more extreme forms of life cycle abbreviations, using only a mollusc to complete their cycle, with or without sexual reproduction. The occurrence of these phenomena among trematode families are reviewed here and show that life cycle truncation has evolved independently many times in the phylogeny of trematodes. The hypotheses proposed to account for life-cycle truncation, in addition to the factors preventing the adoption of shorter cycles by all trematodes are also discussed. The study of shorter life cycles offers an opportunity to understand the forces shaping the evolution of life cycles in general.     

The structure of the muscular and nervous systems of the male Intoshia linei (Orthonectida)

Orthonectida is a small group of parasites, which, according to recent studies, may be phylogenetically close to Annelida. Here, we describe the musculature and serotonin-like immunoreactive (SLIR) nervous system of male adults of Intoshia linei (Orthonectida) using immunohistochemistry and confocal laser scanning microscopy. The whole muscular system consists of four outer longitudinal and eight pairs of inner semicircular muscle fibres. Immunohistochemistry revealed six serotonin-like cells at the anterior part of the body, and two backward lateral longitudinal nerves, merging at the posterior end. Compared to females, the organization of the nervous system is modified and its progenetic origin seems unlikely. The general neuromuscular organization corresponds to the pattern of small-sized annelids, suggesting their possible phylogenetic affinity. Free-living males and females of the orthonectid I. linei may present a good example of a highly simplified Bilaterian with fully functioning nervous and muscular systems. This simplicity is secondary and is caused by two factors—the parasitic life style and miniaturization of free-living sexual stages.     

Why reproduce sexually?

There is reason to believe that intense selection, such that only a small minority at the top of the fitness distribution has any appreciable chance of survival, can sometimes give sexual reproduction an immediate (one-life-cycle) advantage over asexual. The advantage must be great enough to balance the 50% loss of genetic material in meiosis.One model shows the advantage to be frequency-dependent in life cycles in which there are several asexual generations and one sexual. The observed frequency of sexual reproduction in such a life cycle is explained as an evolutionary equilibrium by this model. In another model the optimum frequency of asexual reproduction drops to zero as fecundity and competition increase. This explains the exclusively sexual reproduction of such fecund organism as elms and oysters. Once lost, asexual reproduction may be difficult to evolve secondarily. This explains the presence of such exclusively sexual, low-fecundity organisms as the higher vertebrates.     

HOST IMMUNE STATUS DETERMINES SEXUALITY IN A PARASITIC NEMATODE

We examine the hypothesis that sexual reproduction by parasites is an adaptation to counter the somatic evolution of vertebrate immune responses. This is analogous to the idea that antagonistic coevolution between hosts and their parasites maintains sexual reproduction in host populations. Strongyloides ratti is a parasitic nematode of rats. It can have a direct life cycle, with clonal larvae of the wholly parthenogenetic parasites becoming infective, or an indirect life cycle, with clonal larvae developing into free-living dioecious adults. These free-living adults produce infective larvae by conventional meiosis and syngamy. The occurrence of the sexual cycle is determined by both environmental and genetic factors. By experimentally manipulating host immune status using hypothymic mutants, corticosteroids, whole-body γ-irradiation and previous exposure to S. ratti, we show that larvae from hosts that have acquired immune protection are more likely to develop into sexual adults. This effect is independent of the method of manipulation, larval density, and the number of days postinfection. This immune-determined sexuality is consistent with the idea that sexual reproduction by parasites is adaptive in the face of specific immunity, an idea which, if true, has clinical and epidemiological consequences.     

Sex determination in the parasitic nematode Strongyloides ratti

The parasitic nematode Strongyloides ratti reproduces by both parthenogenesis and sexual reproduction, but its genetics are poorly understood. Cytological evidence suggests that sex determination is an XX/XO system. To investigate this genetically, we isolated a number of sex-linked DNA markers. One of these markers, Sr-mvP1, was shown to be single copy and present at a higher dose in free-living females than in free-living males. The inheritance of two alleles of Sr-mvP1 by RFLP analysis was consistent with XX female and XO male genotypes. Analysis of the results of sexual reproduction demonstrated that all progeny inherit the single paternal X chromosome and one of the two maternal X chromosomes. Therefore, all stages of the S. ratti life cycle, with the exception of the free-living males, are XX and genetically female. These findings are considered in relation to previous analyses of S. ratti and to other known sex determination systems.     

How a complex life cycle can improve a parasite's sex life

How complex life cycles of parasites are maintained is still a fascinating and unresolved topic. Complex life cycles using three host species, free-living stages, asexual and sexual reproduction are widespread in parasitic helminths. For such life cycles, we propose here that maintaining a second intermediate host in the life cycle can be advantageous for the individual parasite to increase the intermixture of different clones and therefore decrease the risk of matings between genetically identical individuals in the definitive host. Using microsatellite markers, we show that clone mixing occurs from the first to the second intermediate host in natural populations of the eye-fluke Diplostomum pseudospathaceum. Most individuals released by the first intermediate host belonged to one clone. In contrast, the second intermediate host was infected with a diverse array of mostly unique parasite genotypes. The proposed advantage of increased parasite clone intermixture may be a novel selection pressure favouring the maintenance of complex life cycles.     

Typology of life cycles of ground beetles (Coleoptera,Carabidae) in Western Palaearctic

An original classification of the life cycles of ground beetles from Western Palaearctic is proposed. The classification is based on a combination of five criteria: duration, number of generations per season, phenology of reproduction, stability, and repeatability of reproduction. According to the individual lifespan, the cycles are subdivided into annual and biennial ones. The annual life cycles may be uni-and bivoltine, whereas biennial ones are always univoltine. By the time of reproduction, winter-spring, spring, spring-summer, early summer, summer, late summer, summer-autumnal, autumnal, autumn-winter, winter, and aseasonal species are distinguished. The biennial and bivoltine cycles may be of both facultative and obligate nature. Species living only one season and having a continuous reproductive period are designated as semelparous, while those breeding during two or more years or having several distinct periods of reproduction in one season, as iteroparous. By now, 30 variants of life cycles in Carabidae from western Palaearctic have been established. Repeated similarly directed modifications of the life cycle may produce essentially different seasonal rhythms in some individuals. In this case, two subpopulation groups usually appear within the population. Under the most unfavorable conditions, these groups become practically isolated and hibernate at different ontogenetic stages. The individual development in each of these groups takes two years with the same seasonal rhythm. Among the types considered, only obligate-bivoltine life cycles are always polyvariant, but annual univoltine and obligate-biennial ones are always univariant. The facultative-bivoltine and biennial life cycles may be realized as uni-and polyvariant ones, depending on the environmental conditions.     

Cytological paradoxes in the developmental cycle of the coelenterate Polypodium hydriforme--an intracellular parasite from the oocytes of acipenserid fishes

Successive stages of the embryonic development of Polypodium hydriforme, occurring at the parasitic phase of its life cycle, are considered. The development of a new parasitic generation starts without fertilization, i. e. parthenogenetically. The embryo develops from aberrant binucleate gametes formed in the result of meiosis within entodermal gonads of free-living animals. This type of gametogenesis, earlier considered as spermatogenesis (Raikova, 1961), is now interpreted as oogenesis. A conclusion is drawn about a change of the sexual orientation of the male gonad which becomes a female one in the course of evolution of Polypodium. As to the gonads of free-living animals, which were formerly interpreted as female ones, they seem to be abortive rudimentary organs since they produce no mature sex cells. A long-lasting block of cytokinesis of the 2nd meiotic division, as well as utilization of the polar body of this division as a phorocyte and, later, as a trophamnion, are important adaptations of Polypodium to parasitism. It is the larger nucleus with a voluminous cytoplasm, rather than the smaller nucleus, that becomes here the 2nd polar body. Polypodium differs from other coelenterates by the presence of highly polyploid feeding cells at both the parasitic (the trophamnion, 500 c) and free-living phases of the life cycle (trophocytes in the rudimentary female gonad, 8c-32c).     

Prolonged clonal growth: escape route or route to extinction?

Many plant species have the capability to reproduce sexually as well as clonally. The balance between clonal reproduction and sexual reproduction varies between different species. It was estimated that 66.5% of all central European flora may form independent but genetically identical daughter plants. Also within species there is great variation in the ratio clonal/sexual reproduction. Clonal reproduction can be considered as an alternative life cycle loop that allows persistence of a species in the absence of the ability to complete the normal life cycle (i.e. seed production, germination and recruitment). Plant populations exhibiting prolonged clonal growth have been referred to as 'remnant populations'. A remnant population in general is defined as "a population capable of persistence during extended time periods despite a negative population growth rate (λ<1) due to longlived life stages and life cycles, including loops, that allow population persistence without completion of the whole life cycle". Here we argue that prolonged and nearly exclusive clonal growth through environmental suppression of sexual reproduction can ultimately lead to local sexual extinction and to monoclonal populations of a species, and that this may imply significant consequences for population viability. Especially obligate or mainly outcrossing clonal plant species may be vulnerable for sexual extinction. We argue that the consequences of reduced sexual recruitment in clonally propagating plants may be understudied and underestimated and that a re-evaluation of current ideas on clonality may be necessary.     

Clonal variation of gene expression as a source of phenotypic diversity in parasitic protozoa

Within a cellular clone, individual cells can express different members of a gene family. If the difference in expression is transmitted to daughter cells, 'phenotypic clones' are formed. Such clonal phenotypic variation has evolved independently in phylogenetically distant parasitic protozoa under similar selective pressure: the need for phenotypic diversity at several steps of their life cycle. Here, I review clonal phenotypic variation processes, outline their role in parasite biology and argue that clonal phenotypic variation is complementary to sexual reproduction as a source of phenotypic diversity.     

Cell reproduction cycle of mycoplasma

The cell reproduction cycle of parasitic wall-free bacteria, mycoplasma, is reviewed. DNA replication of Mycoplasma capricolum starts at a fixed site neighboring the dnaA gene and proceeds to both directions after a short arrest in one direction. The initiation frequency fits to the slow speed of replication fork and DNA content is set constant. The replicated chromosomes migrate to one and three quarters of cell length before cell division to ensure delivery of the replicated DNA to daughter cells. The cell reproduction is based on binary fission but a branch is formed when DNA replication is inhibited. Mycoplasma pneumoniae has a terminal structure, designated as an attachment organelle, responsible for both host cell adhesion and gliding motility. Behavior of the organelle in a cell implies coupling of organelle formation to the cell reproduction cycle. Several proteins coded in three operons are delivered sequentially to a position neighboring the previous organelle and a nascent one is formed. One of the duplicated attachment organelles migrates to the opposite pole of the cell before cell division. It is becoming clear that mycoplasmas have specialized cell reproduction cycles adapted to the limited genome information and parasitic life.     

植物生活史型的多样性及动态分析

主要阐述了植物生活史型的基本定义和基本模式。根据植物的生态幅（Ｅｃｏｌｏｇｉｃａｌ　ａｍｐｌｉｔｕｄｅ）、适合度（Ｆｉｔｎｅｓｓ）和能量分配格局将植物生活史型划分出Ｖ生活史型、Ｓ生活史型和ｃ生活史型３个基本类型以及ＶＳ生活史型、ＳＶ生活史型、ｃＳ生活史型、Ｓｃ生活史型等６个具有混合特征的过渡类型。文中分析了权衡（丁ｒａｄｅ—ｏｆｆ）植物生活史各阶段的能量需求,使之合理地进行能量分配,进而使植物生活史型获得最佳的繁殖和存活效益以及最大的适合度的重要性,指出韧生代谢和次生代谢增值物生活史型及其生活史型之间相互转换的密切关系。韧生代谢物质主要用于营养生长,次生代谢物质主要用于促进繁育和拮抗环境胁迫。植物生活史型在特定时空中依生境的连续变化而发生相互转换,呈现出具动态特征的植物生活史型诺。提出了植物生活史型的形成机制,即生境中的资源状况和干扰程度构成了环境筛的径度,进而形成选择压力,以使植物按需分配能量,合成初级代谢产物或次级代谢产物来应对选择压力,形成自身的生态幅和适应对策,最终与生境相互作用过程中表现出的适合度来表征相应的生活史型。还提出了植物生活史型之间相互转化的机制,即每一种植物生活史型均有与该生活史型相对应的生境类型、选择压力、代谢物质和生活史对策,由于时空的连续变化,生境类型也发生过渡性变化,形成过渡类型（ＥＤ、ＤＥ、ＤＦ、ＦＤ）,因而导致选择压力、代谢物质、生活史对策也发生过渡性变化,形成过渡类型ＬＭ、ＭＬ、ＭＨ、ＨＭ、ＫＲ、ＲＫ、ＲＴ、ＴＲ、ＢＰ、ＰＢ、ＰＡ、ＡＰ,最终通过ＶＳ、ＳＶ、ＳＣ、ＣＳ等过渡类型的形成而实现植物生活史型之间的相互转换。文中以高山红景天（Ｒｈｏｄｉｏｌａ　ｓａｃｈａｌｉｎｅｎｓｉｓ）等５种植物生活史型谱为例,分析了各植物生活史型谱的动态特征并指出：Ｖ生活史型的植物因营养体较为发达、寿命较长,且能通过正常的有性生殖繁衍后代,通常都能产生稳定种群;以Ｓ生活史型为主的植物,因台子中含有来自双亲的两套基因,故有性生殖过程能产生较多遗传性不同的后代,使种群的适应环境变化的能力加强,因而容易形成爆发种群;以ｃ生活史型为主的植物,其遗传物质与母体完全相同,故种群适应环境变化的能力较弱,因而容易导致种群濒危。     

Genomic Confirmation of Hybridisation and Recent Inbreeding in a Vector-Isolated Leishmania Population

Although asexual reproduction via clonal propagation has been proposed as the principal reproductive mechanism across parasitic protozoa of the Leishmania genus, sexual recombination has long been suspected, based on hybrid marker profiles detected in field isolates from different geographical locations. The recent experimental demonstration of a sexual cycle in Leishmania within sand flies has confirmed the occurrence of hybridisation, but knowledge of the parasite life cycle in the wild still remains limited. Here, we use whole genome sequencing to investigate the frequency of sexual reproduction in Leishmania, by sequencing the genomes of 11 Leishmania infantum isolates from sand flies and 1 patient isolate in a focus of cutaneous leishmaniasis in the Çukurova province of southeast Turkey. This is the first genome-wide examination of a vector-isolated population of Leishmania parasites. A genome-wide pattern of patchy heterozygosity and SNP density was observed both within individual strains and across the whole group. Comparisons with other Leishmania donovani complex genome sequences suggest that these isolates are derived from a single cross of two diverse strains with subsequent recombination within the population. This interpretation is supported by a statistical model of the genomic variability for each strain compared to the L. infantum reference genome strain as well as genome-wide scans for recombination within the population. Further analysis of these heterozygous blocks indicates that the two parents were phylogenetically distinct. Patterns of linkage disequilibrium indicate that this population reproduced primarily clonally following the original hybridisation event, but that some recombination also occurred. This observation allowed us to estimate the relative rates of sexual and asexual reproduction within this population, to our knowledge the first quantitative estimate of these events during the Leishmania life cycle.     

Sexual reproduction and dimorphism in the pathogenic basidiomycetes

Many fungi in the Basidiomycota have a dimorphic life cycle, where a monokaryotic yeast form alternates with a dikaryotic hyphal form. Most of the dimorphic basidiomycetes are pathogenic on plants, animals or other fungi. In these species, infection of a host appears to be closely linked to both dimorphism and the process of sexual reproduction. Sex in fungi is governed by a specialized region of the genome known as the mating type locus that confers cell-type identity and regulates progression through the sexual cycle. Here we investigate sexual reproduction and lifestyle in emerging human pathogenic yeasts and plant pathogenic smuts of the Basidiomycota and examine the relationship among sex, dimorphism and pathogenesis.     

Reproductive strategy and the life cycle in graptoloid colonies

Urbanek, A. 1990 10 15: Reproductive strategy and the life cycle in graptoloid colonies. Lethaia , Vol. 23, pp. 333–340. Oslo. ISSN 0024–1164.
Graptoloid colonies were clones composed in all probability of hermaphroditic zooids. Their breeding system approaching that of amphicarpic plants, namely distant out crossing combined with selfing, was exceptionally flexible. Moreover, the balanced coexistence of these extreme means of reproduction may be visualized as an evolutionarily stable strategy (ESS). The evolutionary consequences of such a breeding system might have accounted for the overall high rates of graptoloid evolution as well as for the rapid transformations in large populations. Sexual reproduction of graptoloid colonies was complemented by a multiplication through occasional fragmentation of colonies and subsequent regeneration from the fragments. Fragmentation of colonies played an important role in the survival of graptoloid colonies during catastrophic events such as hurricanes and later in recruitment. Fragmentation followed by regeneration, and only later by sexual reproduction of regenerated fragments, constituted the so-called great cycle (GC), whereas the regular course of events initiated by sexual reproduction in an undisturbed (complete) colony resulted in normal colony formation (astogeny) and is termed the small cycle (SC). Thus the adaptive significance of the sexual process in the life cycle is in the restoration of the perfect' pattern of the complete colony, which offered the best hydrodynamic properties and highest fitness. ▭ Graptolithina, Graptoloidea, breeding sysfem, reproductive strategy, fragmentation, life cycle .     

Sex, Meiosis and Multicellularity

The origin and progress of multicellularity, which is one of the crucial steps in the evolution of life, remains unclear and stringent phylogenetic reconstruction of the process is difficult. However, further theoretical considerations of the problem could be useful for the creation of a verifiable hypothesis. Sex as a ubiquitous biological phenomenon is usually considered as something entirely linked with reproduction. This is mostly true for modem multicellular organisms, but at the earliest stage of evolution of eukaryotes it was not so. At that time the sexual process had nothing to do with reproduction, and only later, sex and reproduction merged together.One of the aims of this paper is to consider the sexual process as a likely basis for the establishment of multicellularity and to discuss the early stages of evolution of the multicellularity from this perspective. It is suggested that mitotic reproduction of cells at different stages of the sexual cycle of unicellular ancestors might be the starting points for independent transition to multicellularity in different taxa. Numerous consequences of these transitions, including evolution of bisexuality and development of novel meiotic functions in animals, are discussed.     

SEXUALITY, INCOMPATIBILITY, SIZE VARIATION, AND PREFERENTIAL POLYANDRY IN NATURAL POPULATIONS AND CLONES OF SELLAPHORA PUPULA (BACILLARIOPHYCEAE)

The capitate and rectangular demes of the freshwater epipelic diatom Sellaphora pupula (Kütz.) Mereschk. are dioecious, the first such report for any freshwater diatom. Sexual differentiation, which is probably determined genetically, involves recognition at the cell surface as well as differences in gamete behavior (one gametangium produces an active "male" gamete, the other a passive "female" gamete). In culture, successful sexual reproduction occurs only when compatible clones are mixed. All cells of a clone behave identically in interclonal crosses, being either male or female, regardless of the stage of the life cycle, in contrast to the sequential hermaphroditism of centric diatoms. Males and females have identical frustule morphology. As in other diatoms, there is an upper size threshold for sexual reproduction, below which cells become progressively easier to sexualize. In culture, sexual interactions occur in cells much smaller than those ever seen in natural populations, so that in nature the sexual size range is effectively open. Natural populations almost always contain sexualizable cells; often, most of the cells are below the upper sexual size threshold. Male gametangia are, on average, slightly larger than females in the capitate deme, which may be produced by preferential polyandry, depleting the population of males and making them younger at mating. Rarely, selfing occurs producing zygotes, but these abort before producing initial cells. The sizes of the gametangia and initial cells are correlated but this does not invalidate the use of "cardinal points" of the life cycle in taxonomy. No interbreeding occurs between the rectangular and capitate demes. However, when males of one deme are mixed with females of the other, there is a stimulation of activity, as during the early stages of pairing in compatible intrademic crosses.     

New insights into placozoan sexual reproduction and development

Unraveling animal life cycles and embryonic development is basic to understanding animal biology and often sheds light on phylogenetic relationships. A key group for understanding the evolution of the Metazoa is the early branching phylum Placozoa, which has attracted rapidly increasing attention. Despite over a hundred years of placozoan research the life cycle of this enigmatic phylum remains unknown. Placozoa are a unique model system for which the nuclear genome was published before the basic biology (i.e. life cycle and development) has been unraveled. Four organismal studies have reported the development of oocytes and one genetic study has nourished the hypothesis of sexual reproduction in natural populations at least in the past. Here we report new observations on sexual reproduction and embryonic development in the Placozoa and support the hypothesis of current sexual reproduction. The regular observation of oocytes and expressed sperm markers provide support that placozoans reproduce sexually in the field. Using whole genome and EST sequences and additional cDNA cloning we identified five conserved sperm markers, characteristic for different stages in spermatogenesis. We also report details on the embryonic development up to a 128-cell stage and new ultrastructural features occurring during early development. These results suggest that sperm and oocyte generation and maturation occur in different placozoans and that clonal lineages reproduce bisexually in addition to the standard mode of vegetative reproduction. The sum of observations is best congruent with the hypothesis of a simple life cycle with an alternation of reproductive modes between bisexual and vegetative reproduction.     

Muscle formation in the sexual genetation of Intoshia variabili (Orthonectida)

Muscle formation in Intoshia variabili (Orthonectida) has been studied in the course of development of the sexual (free-living) specimen. The muscle system originates in the early embryogenesis as a distinct continuous layer located between the outer cell layer and the inner cell mass. Later this cell layer disintegrates into separate muscle strips. The presence of a distinct muscle system in Orthonectida and the pattern of its development evidences for placing this group into Triploblastica rather than into Diploblastica.     

Sexual reproduction as a response to H2O2 damage in Schizosaccharomyces pombe.

Although sexual reproduction is widespread, its adaptive advantage over asexual reproduction is unclear. One major advantage of sex may be its promotion of recombinational repair of DNA damage during meiosis. This idea predicts that treatment of the asexual form of a facultatively sexual-asexual eucaryote with a DNA-damaging agent may cause it to enter the sexual cycle more frequently. Endogenous hydrogen peroxide is a major natural source of DNA damage. Thus, we treated vegetative cells of Schizosaccharomyces pombe with hydrogen peroxide to test if sexual reproduction increases. Among untreated stationary-phase S. pombe populations the sexual spores produced by meiosis represented about 1% of the total cells. However, treatment of late-exponential-phase vegetative cells with hydrogen peroxide increased the percentage of meiotic spores in the stationary phase by 4- to 18-fold. Oxidative damage therefore induces sexual reproduction in a facultatively sexual organism, a result expected by the hypothesis that sex promotes DNA repair.