Parthenogenesis

Abstract: The haplodiploid sex determining mechanism in Hymenoptera (males are haploid, females are diploid) has played an important role in the evolution of this insect order. In Hymenoptera sex is usually determined by a single locus, heterozygotes are female and hemizygotes are male. Under inbreeding, homozygous diploid and sterile males occur which form a genetic burden for a population. We review life history and genetical traits that may overcome the disadvantages of single locus complementary sex determination (sl-CSD). Behavioural adaptations to avoid matings between relatives include active dispersal from natal patches and mating preferences for non-relatives. In non-social species, temporal and spatial segregation of male and female offspring reduces the burden of sl-CSD. In social species, diploid males are produced at the expense of workers and female reproductives. In some social species, diploid males and diploid male producing queens are killed by workers. Diploid male production may have played a role in the evolution or maintenance of polygyny (multiple queens) and polyandry (multiple mating). Some forms of thelytoky (parthenogenetic female production) increase homozygosity and are therefore incompatible with sl-CSD. We discuss a number of hypothetical adaptations to sl-CSD which should be considered in future studies of this insect order.

Abstract: The molecular pathways that trigger the initiation of embryogenesis after fertilization in flowering plants, and prevent its occurrence without fertilization, are not well understood1. Here we show in rice (Oryza sativa) that BABY BOOM1 (BBM1), a member of the AP2 family2 of transcription factors that is expressed in sperm cells, has a key role in this process. Ectopic expression of BBM1 in the egg cell is sufficient for parthenogenesis, which indicates that a single wild-type gene can bypass the fertilization checkpoint in the female gamete. Zygotic expression of BBM1 is initially specific to the male allele but is subsequently biparental, and this is consistent with its observed auto-activation. Triple knockout of the genes BBM1, BBM2 and BBM3 causes embryo arrest and abortion, which are fully rescued by male-transmitted BBM1. These findings suggest that the requirement for fertilization in embryogenesis is mediated by male-genome transmission of pluripotency factors. When genome editing to substitute mitosis for meiosis (MiMe)3,4 is combined with the expression of BBM1 in the egg cell, clonal progeny can be obtained that retain genome-wide parental heterozygosity. The synthetic asexual-propagation trait is heritable through multiple generations of clones. Hybrid crops provide increased yields that cannot be maintained by their progeny owing to genetic segregation. This work establishes the feasibility of asexual reproduction in crops, and could enable the maintenance of hybrids clonally through seed propagation5,6.

Abstract: Asex and Evolution: A Very Large-Scale Overview.- The Evolution of the Problem of Sex.- Apomixis: Basics for Non-botanists.- Cytology of Asexual Animals.- A Graphical Approach to Lineage Selection Between Clonals and Sexuals.- Geographical Parthenogenesis: General Purpose Genotypes and Frozen Niche Variation.- Sex and the Red Queen.- Geographical Parthenogenesis: Opportunities for Asexuality.- The Elusive Clone - In Search of Its True Nature and Identity.- Asexual Speciation.- Darwinulid Ostracods: Ancient Asexual Scandals or Scandalous Gossip?.- Parthenogenesis in Oribatid Mites (Acari, Oribatida): Evolution Without Sex.- Bdelloid Rotifers: Progress in Understanding the Success of an Evolutionary Scandal.- Sex Loss in Monogonont Rotifers.- Cyclical Parthenogenesis in Daphnia: Sexual Versus Asexual Reproduction.- Metasexual Stick Insects: Model Pathways to Losing Sex and Bringing It Back.- Thelytoky in Hymenoptera with Venturia canescens and Leptopilina clavipes as Case Studies.- Sex in Parthenogenetic Planarians: Phylogenetic Relic or Evolutionary Resurrection?.- Sperm-Dependent Parthenogenesis and Hybridogenesis in Teleost Fishes.- Masked Damage: Mutational Load in Hemiclonal Water Frogs.- Lost Sex in the Reptiles: Constraints and Correlations.- An Apomixis-Gene's View on Dandelions.- Allelic Sequence Divergence in the Apomictic Boechera holboellii Complex.- Asexual Reproduction in Infectious Diseases.- What's in a Clone: The Rapid Evolution of Aphid Asexual Lineages in Relation to Geography, Host Plant Adaptation and Resistance to Pesticides.- Epigenetic Mechanisms in Mammals and Their Effects on Cloning Procedures.- Grapevine (Vitis ssp.): Example of Clonal Reproduction in Agricultural Important Plants.

Abstract: The dominance of the diploid state in higher organisms, with haploidy generally confined to the gametic phase, has led to the perception that diploidy is favored by selection. This view is highlighted by the fact that no known female organism within the Metazoa exists exclusively (or even for a prolonged period) in a haploid state. We used fluorescence microscopy and variation at nine microsatellite loci to show that the false spider mite, Brevipalpus phoenicis, consists of haploid female parthenogens. We show that this reproductive anomaly is caused by infection by an undescribed endosymbiotic bacterium, which results in feminization of haploid genetic males.