Occasional" sexuality occurs when a species combines clonal reproduction and genetic mixing. This strategy is predicted to combine the advantages of both asexuality and sexuality, but its actual consequences on the genetic diversity and species longevity are poorly understood. Androgenesis, a reproductive mode in which the offspring inherits its entire nuclear genome from the father, is often reported as a strictly clonal reproductive mode. Androgenesis is the predominant reproductive mode within the hermaphroditic, invasive lineages of the mollusk genus Corbicula. Their ability to reproduce clonally through androgenesis has been determinant in their invasive success, having colonized during the 20th century American and European freshwater systems, where they became notorious invaders with a widespread, global distribution. However, in androgenetic Corbicula clams, occasional genetic mixing between distinct lineages has also been observed when the sperm of one lineage fertilizes the oocyte of another one. Because of these occasional introgressions, the genetic relationships between Corbicula species remained unclear, and the biogeographic origins of the invasive androgenetic lineages have been challenging to identify. To address these issues, we analyzed the patterns of allele sharing for several nuclear and mitochondrial molecular markers among Corbicula individuals collected across both the native and invasive range. Our results show the occurrence of an allelic pool encompassing all Corbicula freshwater species worldwide, including sexual and androgenetic ones, which highlights the substantial genetic mixing within this genus. However, the differences in allele sharing patterns between invasive lineages, and the low diversity within each lineage, suggest recent, distinct biogeographic origins of invasive Corbicula androgenetic lineages. Finally, the polyploidy, high heterozygosity, and hybrid phenotypes and genotypes found in our study probably originated from hybridization events following egg parasitism between distinct Corbicula lineages. This extensive cross-lineage mixing found in Corbicula may generate nuclear diversity in an otherwise asexually reproducing species.

18Corbicula clams were introduced during the 20 th century into America and Europe, where they became 19 notoriously successful invaders with a widespread, global distribution. Their ability to reproduce clonally through 20 androgenesis ("all-male asexuality") has been determinant in their invasive success, with only four invasive clonal 21 lineages detected across Europe and America, one of which is very abundant and widespread on both continents. 22Due to their "all-male asexuality" and egg parasitism between distinct lineages, the evolutionary and geographic 23 origins of the invasive androgenetic lineages have been challenging to identify. We analyzed here the patterns of 24 allele sharing for different molecular markers among Corbicula individuals collected worldwide. We identify three 25 distinct genetic pools containing androgenetic Corbicula lineages. While one sexual Corbicula species forms a 26 distinct fourth genetic pool, the other sexual lineages cluster with the androgenetic ones based on shared alleles. 27One genetic pool contains most androgenetic lineages and sexual C. sandai from Lake Biwa in Japan, pointing to 28 this lake as a likely origin of androgenetic Corbicula lineages. Although three distinct biogeographic origins of 29Corbicula androgenetic lineages have been identified, their recent radiation and cross-lineage genetic mixing 30 hamper classical species delimitation within this clam genus. 31 32

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