1. Optimizing commercial production of triploid Crassostrea virginica through development of elite tetraploid brood stock using cytogenetic techniques
J.T. Sousa and S.K. Allen, Jr.
Aquaculture Genetics and Breeding Technology Center, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062
Notes Change the 3♂ to 3nf for the chart
Make charts black not blue,
Add x’s to the crosses
Change the 10’s in the column, should be in triploid section
Polyploid induction, more specifically the commercial production of triploids and the creation of tetraploid brood stock to support it, has become an important and successful technique in aquaculture of the eastern oyster, Crassostrea virginica. Triploid oysters are valued for their sterility that generates several advantages for oyster culture, as reduced gonadal development, allowing for higher growth rates and superior market quality during the reproductive season [1].
Tetraploid oysters are genetically unique because they are obtained from a triploid x diploid cross, consequently the tetraploid genome is made up of three chromosome sets from the mother (triploid) and one from the father (diploid). Nevertheless, tetraploids undergo reversion, losing entire sets of chromosomes, and become heteroploid mosaics rising from progressive loss of chromosomes from the original polyploid state. The loss of chromosomes from tetraploids and the possible effects of using these mosaic tetraploids for triploid production are of major scientific interest and also a practical concern for commercial oyster culture [2,3].
On a practical level, chromosome loss in tetraploids causes two principal concerns. The first is the fate of future generations of tetraploid brood stock if mosaics are used to create them. Will this further exacerbate chromosome loss by encouraging high levels of aneuploidy in tetraploids? The second and more immediate concern is the fate of the commercial triploid seed produced from mosaic tetraploid brood stock.
Until now, flow cytometry (FCM) was our principal research tool for detecting reversion. FCM data can be rapidly obtained enabling numerous samples, however, there is little information in FCM data about aneuploidy (e.g., hypo- or hyperploid levels). With cytogenetic techniques, we will be able to look at the chromosomes themselves, in order to better understand whether chromosome losses could be explained by differential chromosomal susceptibility or if this is a random process.
We hope to enlighten the hypotheses for reversion in tetraploid eastern oysters and provide valuable information to our tetraploid breeding program.
Background
FCM: fast, accurate and used on a variety of tissues without killing the animal
Little information about aneuploidy
Study the variation in chromosome number in the somatic tissue (gills) and gonad tissue of C. virginica tetraploids
Study the evolution of chromosome loss over time -- evaluate the differences within and between generations
Study the possible negative correlation between aneuploidy and growth, already described in bivalves
Evaluate the differences within and between families on chromosome loss and estimate heritability of the “trait”
Identify the missing chromosomes in order to understand the genetic reasons for chromosome loss.
Objectives
Cytogenetics: More time consuming
Determination of individual chromosome loss a) b)
Fig.1. Flow cytometry histogram of gill cells from female tetraploid C. virginica. The two distinct cell populations visible as peaks, triploid (3N) on the left in red, tetraploid (4N) on the right in yellow, were indicative of many mosaics used as brood stock to produce triploid larvae [1].
Fig.2. Chromosome constitutions in gill cells of C. gigas isolated from tetraploids and heteroploid mosaics. a) A cell from a eutetraploid individual showing 40 chromosomes. b) A hypertriploid cell with 32 chromosomes [4].
References
[1] Allen Jr., S.K., Triploid oysters ensure year-round supply. Oceanus 31, 58–63.
[2] Matt, J.L., Allen Jr., S.K., Heteroploid mosaic tetraploids of Crassostrea virginica produce normal triploid larvae and juveniles as revealed by flow cytometry. Aquaculture 432, 336–345.
[3] Zhang, Q., Yu, H., Howe, A., Chandler, W., Allen Jr, S.K., Cytogenetic mechanism for reversion of triploids to heteroploid mosaics in Crassostrea gigas (Thunberg) and Crassostrea ariakensis. Aquac. Res. 41, 1658–1667.
[4] Zhang, Z., Wang, X., Zhang, Q., Jr, S.A., Cytogenetic mechanism for the aneuploidy and mosaicism found in tetraploid Pacific oyster Crassostrea gigas (Thunberg). J. Ocean Univ. China 13, 125–131.