World Aquaculture Magazine - June 2020

52 JUNE 2020 • WORLD AQUACULTURE • WWW.WA S.ORG The haplodiplontic life cycle of giant kelp can be manipulated to produce sporeless adults that do not have the capability of producing viable offspring. As individual genotypes of gametophytes can be reliably grown in culture in the lab, gametophytes can be screened for nonsense and frameshift mutations that disrupt the function of key genes for fertility in the adult (sporophyte) stage. Early targets for this research are genes that are essential for meiosis. Gametophytes with mutations in matching essential fertility genes can then be crossed, producing a viable adult that is unable to produce haploid spores. These sporeless adults may then be farmed without concerns of invasive species or transgene flow. The Development and Commercialization of Triploid Oysters Ximing Guo Triploids have three sets of chromosomes instead of the two sets found in normal diploids. Because of the extra set of chromosomes, triploids cannot go through meiosis properly, which renders them functionally sterile. Sterility is important for oyster aquaculture because oysters devote more than 50 percent of their tissue weight to gonad production. Mature oysters are basically bags of eggs or sperm, making them undesirable for raw consumption. After spawning, oysters become watery and again lose their appeal. Diploid oysters suffer from poor meat quality during and after the spawning season. Sterile triploids can maintain meat quality and be marketed year-round. In oysters and most bivalve molluscs studied, triploids grow faster than diploids, probably due to their sterility, increased heterozygosity and/or cell size (Guo et al. 2009). Use of sterile triploids also provides genetic containment and prevents interbreeding between cultured stocks and wild populations. The use of sterile triploids is especially important when cultured oysters are non-native such as the Pacific oyster that has been introduced to many continents for aquaculture. Triploid oysters were first produced by inhibiting meiosis in newly fertilized eggs with cytochalasin B (Stanley et al. 1981). Cytochalasin B and other chemicals used for triploid production are toxic and rarely 100 percent effective. Heat and pressure treatments are even less effective. The inability to consistently produce high percentages of triploids has hindered commercial application. Tetraploids with four sets of chromosomes were first developed in the Pacific oyster in 1993 (Guo and Allen 1994a). Tetraploid oysters are fertile and can produced all triploids when mated with diploids (Guo et al. 1996). Triploid oysters produced from diploid × tetraploid mating are 100 percent pure, free of any chemical treatment and genetically superior than induced triploids (less inbreeding). Mated triploids produced from tetraploids grow significantly faster than diploids and chemically induced triploids. The performance of triploid oysters is strongly influenced by the environment and their tetraploid parent. Genetic improvement of tetraploid lines is critical for the production of superior triploid oysters. The development of tetraploid Pacific oysters was followed by rapid commercialization of triploid oysters. Triploid Pacific oysters produced from tetraploids are cultured in many countries worldwide, accounting for 30-40 percent of the production on the West Coast of the US, about 40-50 percent of production in France and 70 percent of the production in China. Tetraploid Eastern oysters were developed in 2001 and also led to rapid commercialization in the US. Now, triploid Eastern oysters account for over 90 percent of the oysters cultured in Chesapeake Bay and 30 percent of the oysters cultured in the northeastern US. Tetraploid induction in other molluscs has been difficult; no breeding population of tetraploids has been obtained and no commercial production of triploids has been possible. Triploid oysters have demonstrated superior growth (Fig. 4), improved meat quality and greatly reduced gonadal development but they are not completely sterile. Triploid oysters do produce some mature gametes, albeit significantly fewer than diploids. First- generation tetraploid oysters were produced by inhibiting polar body I in eggs from triploids fertilized with normal haploid sperm (Guo and Allen 1994a). Without intervention, gametes from triploids produce aneuploids that mostly die. A few aneuploids and triploids may survive and the reproductive potential of triploid oysters (or odds surviving to the next generation) is about 0.0008 percent of diploids (Guo and Allen 1994b). Furthermore, 90 percent of progeny from triploid × triploid mating are triploids. Thus, triploid oysters are mostly sterile but their sterility is not 100 percent. Production and Performance of Tetraploid and Intercross Triploid Rainbow Trout Gregory Weber, Mark Hostuttler, Megan Kirchgessner, Zachary Wright, Timothy Leeds and Brian Beers Triploidy is the only commercially practiced method of sterilization used in salmonids. Triploidy is induced in the great majority of cases by a pressure or temperature shock to embryos administered about 30 minutes following the initiation of fertilization, causing the retention of the second polar body. An alternative approach to producing triploid animals is to cross a normal diploid parent with a tetraploid parent. Tetraploid induction rates of ~80 percent are routinely achieved in rainbow trout Oncorhynchus mykiss when eggs are exposed to 9000 PSI for 8 minutes starting at 62-65 percent of the first cleavage interval FIGURE 4. Triploid (left) and diploid (right) Pacific oysters. Photo: Ximing Guo.