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Cytoplasmic Elements (cytoplasmic + element)
Selected AbstractsHAPLODIPLOIDY AS AN OUTCOME OF COEVOLUTION BETWEEN MALE-KILLING CYTOPLASMIC ELEMENTS AND THEIR HOSTSEVOLUTION, Issue 4 2004Benjamin B. Normark Abstract Haplodiploidy (encompassing both arrhenotoky and paternal genome elimination) could have originated from coevolution between male-killing endosymbiotic bacteria and their hosts. In insects, haplodiploidy tends to arise in lineages that rely on maternally transmitted bacteria for nutrition and that have gregarious broods in which competition between siblings may occur. When siblings compete, there is strong selection on maternally transmitted elements to kill males. I consider a hypothetical bacterial phenotype that renders male zygotes effectively haploid by preventing chromosome decondensation in male-determining sperm nuclei. By causing high male mortality, such a phenotype can be advantageous to the bacterial lineage. By eliminating paternal genes, it can also be advantageous to the host female. A simple model shows that the host female will benefit under a wide range of values for the efficiency of resource re-allocation, the efficiency of transmission, and the viability of haploid males. This hypothesis helps to explain the ecological correlates of the origins of haplodiploidy, as well as such otherwise puzzling phenomena as obligate cannibalism by male Micromalthus beetles, reversion to diploidy by aposymbiotic male stictococcid scale insects, and the bizarre genomic constitution of scale insect bacteriomes. [source] PERSPECTIVE: MATERNAL KIN GROUPS AND THE ORIGINS OF ASYMMETRIC GENETIC SYSTEMS,GENOMIC IMPRINTING, HAPLODIPLOIDY, AND PARTHENOGENESISEVOLUTION, Issue 4 2006Benjamin B. Normark Abstract The genetic systems of animals and plants are typically eumendelian. That is, an equal complement of autosomes is inherited from each of two parents, and at each locus, each parent's allele is equally likely to be expressed and equally likely to be transmitted. Genetic systems that violate any of these eumendelian symmetries are termed asymmetric and include parent-specific gene expression (PSGE), haplodiploidy, thelytoky, and related systems. Asymmetric genetic systems typically arise in lineages with close associations between kin (gregarious siblings, brooding, or viviparity). To date, different explanatory frameworks have been proposed to account for each of the different asymmetric genetic systems. Haig's kinship theory of genomic imprinting argues that PSGE arises when kinship asymmetries between interacting kin create conflicts between maternally and paternally derived alleles. Greater maternal than paternal relatedness within groups selects for more "abstemious" expression of maternally derived alleles and more "greedy" expression of paternally derived alleles. Here, I argue that this process may also underlie origins of haplodiploidy and many origins of thelytoky. The tendency for paternal alleles to be more "greedy" in maternal kin groups means that maternal-paternal conflict is not a zero-sum game: the maternal optimum will more closely correspond to the optimum for family groups and demes and for associated entities such as symbionts. Often in these circumstances, partial or complete suppression of paternal gene expression will evolve (haplodiploidy, thelytoky), or other features of the life cycle will evolve to minimize the conflict (monogamy, inbreeding). Maternally transmitted cytoplasmic elements and maternally imprinted nuclear alleles have a shared interest in minimizing agonistic interactions between female siblings and may cooperate to exclude the paternal genome. Eusociality is the most dramatic expression of the conflict-reducing effects of haplodiploidy, but its original and more widespread function may be suppression of intrafamilial cannibalism. In rare circumstances in which paternal gene products gain access to maternal physiology via a placenta, PSGE with greedy paternal gene expression can persist (e.g., in mammals). [source] CYTO-NUCLEAR EPISTASIS: TWO-LOCUS RANDOM GENETIC DRIFT IN HERMAPHRODITIC AND DIOECIOUS SPECIESEVOLUTION, Issue 4 2006Michael J. Wade Abstract We report the findings of our theoretical investigation of the effect of random genetic drift on the covariance of identity-by-descent (ibd) of nuclear and cytoplasmic genes. The covariance in ibd measures of the degree to which cyto-nuclear gene combinations are heritable, that is, transmitted together from parents to offspring. We show how the mating system affects the covariance of ibd, a potentially important aspect of host-pathogen or host-symbiont coevolution. The magnitude of this covariance influences the degree to which the evolution of apparently neutral cytoplasmic genes, often used in molecular phylogenetics, might be influenced by selection acting on unlinked nuclear genes. To the extent that cyto-nuclear gene combinations are inherited together, genomic conflict is mitigated and intergenomic transfer it facilitated, because genes in both organelle and nuclear genomes share the same evolutionary fate. The covariance of ibd also affects the rate at which cyto-nuclear epistatic variance is converted to additive variance necessary for a response to selection. We find that conversion is biased in species with separate sexes, so that the increment of additive variance added to the nuclear genome exceeds that added to the cytoplasmic genome. As a result, the host might have an adaptive advantage in a coevolutionary arms race with vertically (maternally) transmitted pathogens. Similarly, the nuclear genome could be a source of compensatory mutations for its organellar genomes, as occurs in cytoplasmic male sterility in some plant species. We also discuss the possibility that adaptive cytoplasmic elements, such as favorable mitochondrial mutations or endosymbionts (e.g., Wolbachia), have the potential to release heritable nuclear variation as they sweep through a host population, supporting the view that cytoplasmic introgression plays an important role in adaptation and speciation. [source] Widespread hermaphroditism in freshwater gastrotrichsINVERTEBRATE BIOLOGY, Issue 4 2001Mitchell J. Weiss Abstract. Freshwater members of the phylum Gastrotricha were long thought to lack male gametes and to exist exclusively as parthenogenetic females. The surprising 1978 discovery of sperm in the common species Lepidodermella squamata raised the question of how many other freshwater gastrotrichs might likewise be hermaphroditic. In a comparative study of species from across both major families, sperm have been found in every species examined intensively. They were detected in 19 species of Chaetonotidae (from Aspidiophorus, Chaetonotus, Heterolepidoderma, Ichthydium, Lepidodermella, and Polymerurus) and 3 species of Dasydytidae (from Haltidytes, Setopus, and Stylochaeta), characteristically occurring ventrally in single unilateral or (more often) 2,12 bilateral packets. Their shape ranges from filiform (length in Chaetonotus bisacer, ,40 ,m) to rodlike, spindlelike, oval, and possibly spherical (some in Stylochaeta scirtetica measure only 1 ,m). With light microscopy, a dense nucleus appears to fill the entire volume of these aflagellate cells. Spermatogenesis within cysts (maximally, 16 sperm/clone) is evidently characteristic of both families, each cyst generating one large residual body. Sperm-bearers display oocytes with sometimes distinctive cytoplasmic elements and a posterior X-organ whose organization can be complex. Evidence supports an unusual life cycle in which parthenogenesis is followed by simultaneous hermaphroditism. These findings may illuminate the reproductive characters as well as ancestry of marine and brackish-water taxa of Chaetonotida. [source] |