Hermaphroditic Freshwater Clams in the Genus Corbicula Produce Non-Reductional Spermatozoa With Somatic DNA Content

Abstract

Hermaphroditic freshwater clams in the genus Corbicula produce non-reductional spermatozoa. The DNA content of spermatozoa was almost identical with that of somatic cells in C. leana from Mie Prefecture, Japan. Hermaphroditic C. aff. fluminea from Saga Prefecture and C. fluminea from Taiwan also produce non-reductional spermatozoa. On the other hand, spermatozoa of the dioecious C. sandai had half the DNA found in somatic cells. Analysis of chromosome numbers suggests that C. leana (3n = 54 in somatic cells and 18 in meiotic cells) from Mie Prefecture and C. aff. fluminea (2n = 36 in gills and 18 bivalents in meiotic cells) from Saga Prefecture are triploids and diploids, respectively. C. leana, C. aff. fluminea, and C. fluminea may lack either first or second meiosis, resulting in non-reductional spermatozoa. We assume that gynogenetic reproduction occurs in both species; maternal chromosomes are also nonreductional, and spermatozoa activate development of the eggs, but do not contribute to the offspring.

Androgenesis: a review through the study of the selfish shellfish Corbicula spp.

“…However, lacustrine species of Corbicula from Sulawesi and Sumatra in Indonesia seem dioecious and sexual, but incubate their larvae . All other freshwater Corbicula clams are hermaphroditic and brood their larvae in their gills (Komaru et al, 1997;Byrne et al, 2000;Qiu et al, 2001;Glaubrecht et al, 2006).…”

“…Several hermaphroditic freshwater species of Corbicula produce unreduced spermatozoa: that is, sperm cells containing the same amount of DNA as somatic cells (Komaru et al, 1997, Houki et al, 2011see Table 2). On the other hand, the dioecious, sexually reproducing Corbicula species produce reduced sperm: the DNA content of spermatozoa is half of that of somatic cells.…”

“…Apomictic production of diploid (or polyploid) sperm derives from mutations in the meiotic mechanism that arise quite frequently in animals (reviewed in McKone and Halpern, 2003). Automictic production of unreduced sperm could occur through several proposed mechanisms, and each of these will have different expectations for the genetic consequences: (i) pre-meiotic doubling of DNA during spermatogenesis (Komma and Endow, 1995;Haccou and Schneider, 2004); (ii) fusion of sperm after meiosis (McKone and Halpern, 2003) and (iii) abortive cytokinesis during meiosis (Komaru et al, 1997). Any viable hypothesis for the production of unreduced sperm in Corbicula must consider both the maintenance of not just diploid, but triploid (and maybe tetraploid) sperm, and the observation of heterozygosity across nuclear loci (Hedtke et al, 2008;Pigneur et al, 2011b).…”

“…Further examination of spermatogenesis will be a critical next step in determining the genetic consequences of androgenesis, as sperm production is the time in the life cycle of androgenetic Corbicula where recombination or segregation could potentially occur. Komaru et al (1997) initially hypothesized that C. leana, C. fluminea and C. aff. fluminea, three species with unreduced sperm, reproduced through pseudogamous gynogenesis.…”

“…Both diploid and triploid (and probably tetraploid) androgenetic Corbicula produce unreduced spermatozoa (Komaru et al, 1997). This suggests that diploid androgenetic individuals may evolve from a diploid ancestral population with normal meiosis (Qiu et al, 2001).…”