Clonal genome evolution and rapid invasive spread of the marbled crayfish

Freshwater crayfish belong to the Order “decapod crustaceans”, which includes crabs, lobsters, prawns and shrimps. Complete genome sequences from these groups remain to be established. Currently, the only crustacean genomes available are those of the water flea (Daphnia pulex) and sand flea (Parhyale hawaiensis). The marbled crayfish Procambarus virginalis is a freshwater animal that is unique among decapod crustaceans because of its parthenogenetic mode of reproduction (form of asexual reproduction in which embryos develop in the absence of fertilization; most commonly found in plants and invertebrate organisms, an increasing number of vertebrate species –– including some forward-thinking individuals in Western Society –– have recently been contemplating this reproductive strategy). There are two methods by which parthenogenesis occurs: one is by apomixis, where egg cells are produced by mitosis (cell division that results in two daughter cells, each having the same number and kind of chromosomes as the parent nucleus); the other is apomictic parthenogenesis –– in which the female sex cell (haploid oocyte, having a single set of chromosomes) replicates by mitosis, producing two diploid cells (having pairs of chromosomes) –– and these cells have the full complement of chromosomes needed to develop into an embryo.

Marbled crayfish are descendants of the sexually-reproducing slough crayfish Procambarus fallax and reproduce by apomictic parthenogenesis. Authors had previously postulated that marbled crayfish might have originated through an evolutionarily very recent macro-mutation in P. fallax, consistent with the first known appearance of marbled crayfish in the German aquarium trade in the mid-1990s. Subsequent distribution –– via the household pet trade and anthropogenic releases of crayfish into the wild –– has resulted in increasing numbers of wild populations in several countries. Propagation of marbled crayfish is facilitated by their parthenogenetic mode of reproduction and their high fecundity (high rate of fertility, plus large numbers of offspring), which allows establishment of large populations from single animals, and may serve as a model for the spread of invasive species. However, our understanding of marbled crayfish distribution, origins, diversification, and ability to adapt to new environments has been severely limited by the lack of genetic information –– until now.

Authors [see attached] have now determined a draft assembly of the marbled crayfish genome. The genome size is ~3.5 gigabase-pairs (Gbp) and identified >21,000 genes (both size and gene number are in the same range as that of human). Authors also confirmed the close relationship of the marbled crayfish genome to the genome of the slough crayfish, and discovered a triploid AA’B genotype (having three sets of chromosomes) with a high level of heterozygosity (having different alleles at the same genetic locus). Comparative whole-genome sequencing demonstrated the clonality of the population and their genetic identity with the oldest known stock from the German aquarium trade. These fascinating data contribute important knowledge to the phylogenetic analysis of animal genomes and uncover the unique evolutionary history of an emerging invasive species.

Nat Ecol Evol 5 Feb 2o18; doi: 10.1038/s41559-018-0467-9.

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