Corbicula

Androgenesis occurs rarely in the tree of life, with only a few occurrences documented and multiple mechanisms through which it is achieved. In Corbicula, androgenesis occurs through the ejection of maternal chromosomes from the egg following fertilization by sperm.^[4] Fertilization in androgenetic and sexual lineages of Corbicula occurs while the developing egg is arrested at metaphase 1 of meiosis. In sexual species of Corbicula, the axis of the meiotic spindle orienting the duplicated maternal chromosomes is perpendicular to the plasma membrane of the zygote. However, androgenetic lineages of Corbicula have an axis parallel to the membrane. As a result of this unusual orientation, the two maternal polar bodies formed during anaphase 1 are extruded from the zygote, leading to the complete elimination of all maternal chromosomes.^[5][6] Androgenetic Corbicula lineages also have unreduced sperm; therefore, these lineages retain the same ploidy level after maternal chromosome extrusion. Since only maternal chromosomes are eliminated from the zygote, the zygote inherits only the paternal genome. Sperm of sexually reproducing Corbicula are uniflagellate, which is considered the ancestral trait, while androgenetic Corbicula lineages interestingly possess biflagellate sperm.^[4]

While androgenesis would likely lead to species extinction in dioecious species,^[7] all androgenetic lineages of Corbicula are hermaphroditic, meaning individuals can produce both sperm and egg, and these individuals can self-fertilize to create effectively clonal offspring. Androgenetic lineages of Corbicula are capable of cross-breeding with sexual and other androgenetic lineages in a phenomenon known as “egg parasitism”.^[8] This leads to several interesting consequences for determining androgenetic Corbicula phylogeny. The first is a “cytonuclear mismatch” whereby the mitochondrial DNA shows congruence with the parasitized lineage but the genomic DNA is congruent with the selfish androgenetic lineage whose sperm fertilized the egg. Further complicating phylogenetic studies is the rare occurrence of partial or complete nuclear capture, when the maternal DNA is not completely eliminated from the zygote. Nuclear capture can result in genome recombination or polyploidy. Partial genome capture has been documented when native and androgenetic or multiple androgenetic lineages are sympatric. Egg parasitism has been offered as one explanation for the persistence of androgenetic lineages through increasing allele heterozygosity.^[9]

Despite extensive phylogenetic study of the genus, appropriate categorization of invasive populations has remained a challenge.^[10][9][11] Lack of clarity in their phylogeny may be due to being hermaphroditic androgens,^[12][13] though no single species of Corbicula has been described as fully androgenetic. Rather, 4-5 specific androgenetic lineages are described in the scientific literature. Form A, B, and D are found within the North America;^[12][14] Form C is in South America;^[15][12] and another form(s) has described in Europe.^[9][11] Cross-breeding between androgenetic and native Corbicula lineages have made it difficult to create a clear taxonomy of the genus, and it is still unclear whether androgenesis arose independently multiple times or originated from a smaller number of lineages that then cross-bred with sexual Corbicula species.^[16][9]

Corbicula clams are remarkably proficient invasive species, with native ranges spanning from Australia to Africa, but can now be found in most other continents.^[10] In North America, Corbicula may have initially invaded as a human food source,^[17] though the origin of invasion in other continents has not been determined.^[18] However, genotyping may aid in tracking the number of introductions occurring in non-native habitats.^[19]

Part of what contributes to its invasive success is its androgenetic reproductive strategy, wherein a single individual may be capable of creating an entire population,^[10] but beyond androgenesis, Corbicula owe their invasive potential to anthropogenic factors and their life history strategies.^[20] Corbicula have high reproductive capacities,^[21] which may be in part due to their ability to self-fertilize,^[10] and the high dispersal potential of their larvae.^{[citation needed]} Corbicula are also phenotypically plastic,^[22][10] which may allow them to outcompete native mussels,^[23] and their occurrence at high densities may drive native mussel glochidia mortality.^[22] Their high competitive ability is of concern, in part due to the already endangered status of many of the world’s mussel species.^[24][25]

Though Corbicula are proficient competitors, they have a small number of lineages,^[12] and have worldwide low genetic diversity, which is attributed to their reproductive capabilities.^[26] While this generally does not contribute to their success, phenotypic plasticity may buffer them from the effects of low genetic diversity,^[10] though it is suggested that population bottlenecks may have occurred during their invasions.^[10][26][27] Despite the potential for population bottlenecks, there is a need for better control methods,^[11] as active spread has occurred.^[28][29] While some eradication methods work, such as deposition of dry ice pellets,^[30] the use of a heat torch,^[31] and temperature shock,^[32] preventative measures are of utmost importance as invasives are often difficult to detect prior to establishment.^[11]

Extant species within the genus Corbicula include:^[2]

• Bogan, A., Bouchet, P. (1998). Cementation in the freshwater bivalve family Corbiculidae (Mollusca: Bivalvia): a new genus and species from Lake Poso, Indonesia. Hydrobiologia, 389: 131-139
• Suzuki, K.; Oyama, K. (1943). Überblick über die Corbiculiden Ostasiens (Materialien zur Monographic der Ostasiatischen Corbiculiden 1). Venus. 12(3-4): 138–149.
• Ota, Y. [Ohta, Y.]. (1970). A review of some Cretaceous corbiculids in North America. Transactions and Proceedings of the Palaeontological Society of Japan, new series. 79: 291–315.

• Alexei V. Korniushin, Matthias Glaubrecht (2003) Novel reproductive modes in freshwater clams: brooding and larval morphology in Southeast Asian taxa of Corbicula (Mollusca, Bivalvia, Corbiculidae) Acta Zoologica 84 (4), 293–315. https://doi.org/10.1046/j.1463-6395.2003.00150.x
• (Redescription) Coan, E. V.; Valentich-Scott, P. (2012). Bivalve seashells of tropical West America. Marine bivalve mollusks from Baja California to northern Peru. 2 vols, 1258 pp.

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• von Mühlfeld J.C. (1811). Entwurf eines neuen Systems der Schaltiergehäuse. Magazin für die neuesten Entdecklungen in der gesammten Naturkunde von der Gesellschaft Naturforschaft Freunde zu Berlin. 5(1): 38-72, plate 3
• Dall, W. H. (1903). Review of the classification of the Cyrenacea. Proceedings of the Biological Society of Washington. 16: 5-8.
• Lindholm W.A. (1933). Eine verschollene Muschel aus Zentralasien. Archiv für Molluskenkunde. 65: 264–268
• Sandberger, C.L.F. (1870-1875). Die Land- und Süßwasser-Conchylien der Vorwelt. C. W. Kreidel, Wiesbaden.