Biomphalaria have an external shell of calcium carbonate covered by a thin cuticle of organic material, which imparts the shell color. Attached internally to the shell in one or more places, a strong muscle allows the snail to draw the head and foot completely inside the shell to escape danger. The snail shell gradually increases in diameter from the blind end (the apex) toward the open end (the aperture). The tube is usually coiled, and the shells of Biomphalaria turn counterclockwise (sinistral or left-handed coil). The strain being sequenced, BB02, may weigh between 0.2- 0.4 gram at maturity.
Biomphalaria glabrata is hermaphroditic (reproduces by both self- and cross-fertilization), however, the preferred reproductive mechanism for this organism is cross-fertilization. Having the ability to self-fertilize surely plays a role in the success of colonization and re-colonization of habitats. Both egg and sperm are produced in a single organ, but the mature egg and sperm are dispersed through separate ducts, the oviduct and the sperm duct. On a typical laboratory diet (romaine lettuce), a non-infected snail has a lifespan of approximately 9-12 months, and it can be expected to produce multiple generations over a year's time. A 4-6 week generation time (egg to egg) is the norm. Parasite infection of these snails has been shown to affect the snail's reproductive capacity, especially in egg production. This phenomenon, known as parasitic castration, is thought to involve hormones released as a result of the parasite infection. A similar reduction in snail reproduction has been shown to occur in those snails that are resistant to infection, when they are raised in the same environment as their susceptible counterparts. This reduced reproductive capacity is thought to be a “trade-off” feature in snails that are resistant. Age is another feature that affects the snail's response to infection, with juveniles being more easily infected than adult snails. Some snails, at sexual maturation, produce nodules in the tissue wall surrounding the heart. This is a blood-forming tissue that produces blood cells called hemocytes, and these cells are intimately involved in the ability of the snail to ward off parasitic infection. Aiding this phenomenon are components of the snail plasma or hemolymph. This organism should lend interesting insights into the host-parasite relationship, known to be highly complex, and it is hoped that this knowledge ultimately could lead to the development of a protective vaccine against the parasitic infection in humans.
-- Courtesy of Matty Knight, a white paper author; also adapted from Scientific Publication No. 168, October 1968, Pan American Health Organization
Freshwater snails of the genus Biomphalaria are intermediate snail hosts for the transmission of the medically important trematode parasite, Schistosoma mansoni, the causative agent of the human tropical disease schistosomiasis, commonly known as, Bilharzia, a disease that afflicts 75 countries in the developing world. Of the 7 species of Biomphalaria transmitting schistosomiasis in the Western Hemisphere, B. glabrata, a highly derived member of the class Gastropoda, is the most important and the best studied experimentally. It is found mostly in South America and the Greater and Lesser Antilles where the snails occupy habitats that are often temporary due to frequent floods and droughts. They are dispersed to new habitats during times of flooding.
With the advent of next generation sequencing technologies, a revised sequencing plan for Biomphalaria glabrata was submitted to the NGHRI and was approved. It will consist of 1X coverage from plasmids on the ABI3730xl robots to provide a genome framework with paired end data. There will be 10X coverage performed on the Roche 454 Sequencer, which will include 2 paired end read runs to increase the linking information gained from the plasmids and 0.1X BAC end sequencing. BAC filters (Arizona Genomics Institute) are available to the research community for probing and identification of additional targets. Twelve BACs have been chosen interactively with the research community for sequence improvement to optimize the immediate scientific benefit of the sequence data. Roche 454-based cDNA sequencing to collect as many full-length cDNAs as possible will be done to provide immediate access to gene models.
The Wellcome Trust Sanger Institute will produce ~40,000 ESTs from conventional libraries provided by the community. If the libraries are of sufficiently high quality, the WTSI will sequence additional ESTs. The combined data from conventional ESTs and 454 cDNA sequences will provide the research community with an excellent resource for gene discovery, annotation and development of microarrays.
For more information, see the VectorBase page.