Background Achatina fulica, the giant African snail, is the largest terrestrial mollusk species. Owing to its voracious appetite, wide environmental adaptability, high growth rate, and reproductive capacity, it has become an invasive species across the world, mainly in Southeast Asia, Japan, the western Pacific islands, and China. This pest can damage agricultural crops and is an intermediate host of many parasites that can threaten human health. However, genomic information of A. fulica remains limited, hindering genetic and genomic studies for invasion control and management of the species.FindingsUsing a k-mer–based method, we estimated the A. fulica genome size to be 2.12 Gb, with a high repeat content up to 71%. Roughly 101.6 Gb genomic long-read data of A. fulica were generated from the Pacific Biosciences sequencing platform and assembled to produce a first A. fulica genome of 1.85 Gb with a contig N50 length of 726 kb. Using contact information from the Hi-C sequencing data, we successfully anchored 99.32% contig sequences into 31 chromosomes, leading to the final contig and scaffold N50 length of 721 kb and 59.6 Mb, respectively. The continuity, completeness, and accuracy were evaluated by genome comparison with other mollusk genomes, BUSCO assessment, and genomic read mapping. A total of 23,726 protein-coding genes were predicted from the assembled genome, among which 96.34% of the genes were functionally annotated. The phylogenetic analysis using whole-genome protein-coding genes revealed that A. fulica separated from a common ancestor with Biomphalaria glabrata ∼182 million years ago.ConclusionTo our knowledge, the A. fulica genome is the first terrestrial mollusk genome published to date. The chromosome sequence of A. fulica will provide the research community with a valuable resource for population genetics and environmental adaptation studies for the species, as well as investigations of the chromosome-level of evolution within mollusks.

Abstract. Two internal transcribed spacer (ITS1 and ITS2) sequences of the non-coding region of the rDNA and a 16S RNA gene fragment of the mtDNA of Oncomelania hupensis, the intermediate host snail of Schistosoma japonicum in mainland China, have been investigated with a view to illustrate the influence of the environment on genetic differentiation. Thirteen populations of O. hupensis snails, representing four types of ecological settings, were collected to compare genetic and spatial distances. The length of the ITS1-5.8S-ITS2 gene sequence showed a range from 752 to 796 bp with a guanine-cytosine (GC) content of 49.1-50.3%, while that of the 16S sequence ranged from 505 to 508 bp with a GC content of 33.5-35.1%. The 5.8S fragment was shown to be highly conserved and it was therefore removed in the subsequent analysis. In contrast, the ITS flanking sequences and the 16S fragment were found useful for further study as their degree of polymorphism amounted to 13.2% and 8.6%, respectively. The genetic relationship was investigated using tools based on maximum parsimony, minimum evolution and neighbour-joining algorithms. Four branches of O. hupensis were found to be clearly represented on the 16S phylogenetic tree, namely (i) the mountainous region population (from Sichuan and Yunnan provinces); (ii) the Karst region population (from Guangxi autonomous region); (iii) the population representing the region of swamps and lakes along the Yangtze River basin which stretches through the Anhui, Hunan, Hubei, Jiangxi, Jiangsu and Zhejiang provinces; and (iv) the littoral, hilly region population (from Fujian province). These four population branches were found to be correlated to environmental features based on the concept of landscape ecology attributing genetic differentiation to differences in ecological features. However, only three main branches could be found on the ITS1-ITS2 phylogenetic tree. The swamps and lakes population (from the Yangtze River basin) and the littoral, hilly population (from Fujian province) were clustered on the third branch in spite of these two populations not being spatially related, i.e. no firm genetic demarcation between their snail populations was found. Thus, it seems that the Fujian population does not constitute a separate branch but belongs to the third branch. This fact, together with the strong genetic evidence that the subspecies O. hupensis guangxiensis represents a discrete branch, support the hypothesis that genetic differentiation of O. hupensis in mainland China is ultimately structured by landscape ecology.