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Molecular evidence for parthenogenesis in a crayfish species Procambarus clarkii
Self-cloning (parthenogenesis) is extremely rare in decapods1. We discovered diploid parthenogenesis in an invasive species red swamp crayfish by genotyping 120 wild individuals collected from two locations in China using five microsatellites2. We found that the origin of parthenogenetic clones arose quite recently. The existence of parthenogenesis of this crayfish in the wild will seriously threaten threatened aquatic plants, freshwater ecosystems and aquaculture, and rice field, but provides an opportunity to investigate the environmental factors that induce parthenogenesis.
Red swamp crayfish, Procambarus clarkii native to south-central United States and Northeastern Mexico, has been transplanted to many other countries3. It was introduced to Nanjing, China from Japan in 1930s. This crayfish has spread to most provinces of China, and established dense populations due to translocation by humans for food, and no local predators4. Its production for human consumption reached 150,000 tones in 2005. However, the species threatened local fish, crustacean, aquatic plants, and local freshwater ecosystem. Their burrows lead to the loss of irrigation water and destroy rice field4, 5.
In order to understand the population biology of this invasive species, we collected 120 (69 females and 51 males) adult individuals in two locations (Nanjing, Jiangsu province and Tongxiang, Zhejiang province) in China, and genotyped them using five tetra-nucleotide microsatellites2 as described previously6. We surprisingly found that four genotype groups were shared by six, five, two and two female individuals (clones 1-4), respectively (Table 1), while the remaining 105 individuals each showed a unique genotype group. Re-extraction of DNA from tissues and regenotyping by another person detected the same results, thus the possibility of DNA contamination would be excluded. According to the frequencies of 44 alleles at the five microsatellite loci in the 120 individuals, the possibility of two individuals sharing one genotype group was 1.47 x 10-5. Thus, these 15 individuals sharing the four genotype groups must be generated through self-cloning (parthenogenesis). These 15 individuals were most probably diploid due to presence of one or two alleles per locus in all five loci (Table 1).
To confirm that the parthenogenetic crayfish is the species Procambarus clarkii, we sequenced the mitochondrial gene Cox 1 of all 120 individuals and compared the sequences with those of other closely related crayfish species1. No nucleotide polymorphism was detected in the crayfish collected in China. Phylogenetic analysis7 based on the partial sequenceof Cox 1 indicated that the crayfish was really the species Procambarus clarkii (Fig. 1A and 1B). Phylogenetic analysis using microsatellite allele sharing index8 revealed that the four clones were clustered with ordinary individuals in different branches (Fig. 1C), and no individuals from the two locations shared any genotype group. These data indicate quite recent origin of the four clones. As parthenogenesis in this crayfish in has not yet been observed in other countries, we suspected that parthenogenesis in this crayfish was caused by specific environment factors in China. The discovery of parthenogenesis in this crayfish species provides an opportunity to study the environmental factors that induce parthenogenesis. In addition, parthenogenetic red swamp crayfish could be a good laboratory model as the parthenogenetic marbled crayfish1 for studying development, ecology and evolution.
Parthenogenetic crayfish individuals in the wild will seriously threaten aquatic plants, freshwater ecosystem and aquaculture, and rice field, as a single parthenogenetic individual is enough to found a population. Hence, translocation of this crayfish among different regions must be banned to prevent its spread, and parthenogenetic individuals in the wild must be eradicated in situ.
Please space between paragraphs. (Say: "We are non-native English speakers.)
I don't see the point in mentioning that the crayfish you collected were wild. I would leave "wild" out. (Are there such things as tame crayfish?)
In plain english, what does "We found that the origin of parthenogenetic clones arose quite recently" mean?
I suggest deleting "threatened" in the phrase "threaten threatened aquatic plants".
Say: "Red swamp crayfish...have been transplanted...."
Say: "They were introduced...."
Say: "...and has established dense populations...."
Say: ""due to translocation by humans for food and lack of local predators."
tons (AE) tonnes (BE)
Say: "...the species threatens local fish, crustacean...and local freshwater ecosystems."
Say: "Their burrows...destroy rice fields."
I think I would use frequency there rather than frequencies.
I suspect that you already know that the phrase should be environmental factors (as the second sentence indicates).
I suggest that you delete wild and will.