Thesis (MScAgric)--Stellenbosch University / ENGLISH ABSTRACT: Triploidy is the genetic state of containing three sets of chromosomes per cell in stead of two as in diploid organisms. The South African abalone (Haliotis midae) is naturally a diploid organism that sexually matures between four to eight years of age. Early sexual maturity is a disadvantage in cultured abalone stock, as the process of gonad development and spawning is energy demanding, causing energy to be diverted away from somatic growth. This same problem has been extensively experienced in diploid bivalve molluscs, where triploidy has since been applied as a means to prevent sexual maturation from occurring, thereby speeding up the growth process and shortening the time to marketing. Because triploidy was effective in bivalves, it was thought that it could contribute to faster growth in abalone as well. A procedure for the induction of triploidy in the abalone, Haliotis midae, was developed by De Beer (2004) and yielded up to 100 percent triploidy in treated abalone larvae. The next step was to compare the growth of the diploids and triploids to establish whether there was indeed a growth advantage on the part of the triploids, in view of commercial application. By using the same techniques as described by De Beer (2004), three groups consisting of triploid and diploid siblings were produced and subscribed to a comparative growth trial. The groups were spawned in three different seasons. The main objective was to establish whether there was in fact a difference in growth between diploid and triploid siblings, and whether seasonal effects were associated with growth advantages for either triploids or diploids. The two growth parameters measured were shell length and body weight. Measurements commenced at eight months of age, when the abalone could be individually tagged and continued up to the age of 24 months. The over-all results provided no convincing evidence of statistically significant faster growth of triploid juveniles compared to that of diploids up to two years of age. Growth differences were detected between seasons, but could not confidently be ascribed to seasonal environmental effects. The regression of shell length to body weight was similar for diploids and triploids. / AFRIKAANSE OPSOMMING: Triploiede organismes bevat drie stelle chromosome per sel in plaas van twee soos dit normaalweg in diploiede diere voorkom. Die Suid Afrikaanse perlemoen (Haliotis midae) is van nature ‘n diploiede organisme wat tussen die ouderdom van vier tot agt jaar seksueel aktief word. Vroeë seksuele aktiwiteit is ongewens in kommersiële akwakultuur aangesien energie spandeer word aan gonade ontwikkeling in plaas van somatiese groei. Dieselfde probleem is vroeër in die oester bedryf ondervind waar dit deur middel van triploiede induksie aangespreek is. Triploiedie veroorsaak steriliteit en kan gebruik word as ’n metode om steriliteit op groot skaal te induseer. Steriliteit sou dan meebring dat meer energie beskikbaar is vir somatiese ontwikkeling, wat verhoogde groeitempo en n verkorte tyd tot bemarking beteken. Op soortgelyke wyse is dus gepostuleer dat triploiedie in perlemoen ook tot steriliteit kon lei. ‘n Triploiede induksie metode was ontwikkel deur Mathilde de Beer (2004) wat ‘n hoë persentasie triploidie in geinduseerde perlemoen opgelewer het. Die volgende logiese stap was om die groei van diploiede diere met die van triploiede diere te vergelyk om te bepaal of triploiedie wel ’n groei voordeel tot gevolg het met die oog op kommersiële toepassing. Deur van dieselfde tegnieke as De Beer (2004) gebruik te maak, is drie groepe, elk bestaande uit verwante diploiede en triploiede diere, geproduseer en ingeskryf aan n vergelykende groei proef. Die groepe was in drie verskillende seisoene geproduseer. Die hoof doelstelling van die proef was om groeitempo van diploiede en triploiede diere te vergelyk, asook om die invloed van seisoen op groei van diploide en triploide te bepaal. Twee groei eienskappe naamlik skulp lengte en liggaamsmassa is gemeet vanaf ‘n ouderdom van agt maande (wanneer die diere individueel gemerk kon word) tot ‘n ouderdom van 24 maande. Die algehele resultate het gedui op geen betekenisvolle verskil tussen die groei van triploiede en diploiede perlemoen tot op die ouderdom van twee jaar. Verskille het voorgekom in die groei tussen seisoene, maar daar kon nie bewys word dat die verskille die gevolg van seisoenale omgewingseffekte was nie. Diploiede en triploiede het dieselfde skulp lengte tot liggaamsmassa verhouding getoon tot op twee jaar ouderdom.

There are four members involved in the breeding complexes of poeciliid fishes found in the freshwaters of northeastern Mexico: males and females of a bisexual species, and diploid and triploid unisexuals. Both unisexuals reproduce by gynogenesis, i.e., an asexual type of reproduction where the sperm triggers egg development but the male genome is excluded to produce clonal offspring. The three types of females are closely related, which suggests that they are potential competitors since all three require the service of the same males. The potential for competition is compounded by a highly skewed sex ratio in favor of females. On the average the unisexuals comprise about 30% of the Poecilia females. This high frequency coupled with a close genetic relatedness to their bisexual hosts, raises the question of how the unisexuals are maintained in nature. Other investigators who work with bisexual/unisexual complexes in the related genus, Poeciliopsis, have postulated that male dominance hierarchies are responsible for restricting the access of subordinate males to their conspecific females. Consequently, these subordinate males mate with unisexual females. The current report tests whether or not this hypothesis applies to bisexual/unisexual complexes of Poecilia. We have found that linear dominance hierarchies appear to function in the defense of home ranges and do not restrict access of males to females. Dominant males exhibit less mating activity than subordinate males towards females. Previous reports showed that males are reproductively competent throughout the year, whereas females show striking asynchrony in their reproductive readiness. Such asynchrony limits the proportion of receptive females at any one time. Consequently, there are more males ready to mate than there are females receptive to their mating attempts. This may lead to mating frenzies. We postulate that these indiscriminate matings maintain the fertility of both unisexuals. When the relative reproductive outputs of adult females are compared, both unisexuals appear as fit as their bisexual congeners.

A demographic population simulation model was developed to examine alternative fishery management strategies and their likely effects on the probability of extirpation of local eastern oyster (Crassostrea virginica) populations in the Chesapeake Bay. Management strategies include varying the minimum shell length-at-harvest, harvest rate, and rate and frequency of stocking of oyster seed with respect to varying salinities and oyster population densities. We also examined the rate of disease-mediated mortality that can be tolerated by a viable population. High density populations at low salinity sites remained viable under a 100% harvest rate and 76.6 minimum shell length-at-harvest due to increased fertilization efficiency in high densities, which increased reproduction. Low density populations at low salinity sites remained viable when harvest rate was set at 0.5 and minimum shell length-at-harvest was set at 85 mm. Neither reducing harvest rate nor minimum shell length-at-harvest produced a viable population at high salinity sites. The effects of disease-mediated mortality were too great for these management options to decrease the probability of extirpation to zero. Supplemental stocking conducted regularly reduced extirpation probabilities to zero and pulse stocking (every five to ten years) did as well, although it required a much larger number of oysters to be stocked. Decreasing disease-mediated mortality rates by 20% in high density populations and by 80% in low density populations reduced the probability of extirpation to zero, suggesting the degree of genetic improvement needed to rebuild eastern oyster populations in the Chesapeake Bay. Culture of a non-native species, such as the Suminoe oyster (Crassostrea ariakensis), could supplement harvest of the declining eastern oyster fishery in Chesapeake Bay. Because of possible ecological impacts from introducing a fertile non-native species, introduction of sterile triploid oysters has been proposed. However, recent data show that a small percentage of triploid individuals progressively revert toward diploidy, introducing the possibility Suminoe oyster might establish self-sustaining populations. To assess the risk of Suminoe oyster populations becoming established in Chesapeake Bay, a demographic population model was developed. Inputs modeled included: salinity, stocking density, reversion rate, reproductive potential, natural and harvest mortality, growth rates, and effects of various management strategies, including harvest criteria. Results showed decreased probability of a Suminoe oyster population becoming self-sustaining when oysters are grown at low salinity sites, certainty of harvest is high, minimum shell length-at-harvest is small, and stocking density is low. Results of the model suggest management strategies that will decrease the probability of a Suminoe oyster population becoming self-sustaining. Policy makers and fishery managers can use the model to predict potential outcomes of policy decisions, supporting the ability to make science-based policy decisions about the proposed introduction of triploid Suminoe oysters into the Chesapeake Bay. / Master of Science

Breeding compexes of poeciliid fishes with a bisexual and two unisexual species were studied for mechanisms permitting Sympatric coexistence. The unisexuals are gynogenetic and thereby sexually dependent on the males of the bisexual species for sperm to initiate development, but inheritance is entirely maternal. Bisexual females are more abundant in headwater localities; unisexuals increase in downstream localities. Males were 10 - 18% of the total poeciliid population, regardless of the relative proportions of bisexual to unisexual females. Downstream localities were typified by greater habitat diversity, including a variety of backwater pools. The unisexuals showed a marked preference for such pools. Both field and laboratory studies showed that all three types of females as well as males preferred shaded areas with a gravel substrate. Although the four types of fish were found together, nearest neighbour data indicated that each type of female preferred its own kind. Males courted throughout the year and were indiscriminate in their choice of mates. Despite the skewed sex ratio, males were not in short supply because only a few females were sexually receptive at a given time. No significant differences existed between bisexuals and unisexuals in their relative reproductive outputs, but they were asynchronous.

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