R. Jack Schultz

Role of Polyploidy in the Evolution of Fishes

Polyploidy is not generally believed to have played a major role in the evolution of animals. This view has been fostered principally by G. L. Stebbins and M. J. D. White, both of whom as early as the 1940’s advocated that Polyploidy at best has played a secondary role in evolution; neither of them has substantially altered this view in recent years (1,2). Some of the arguments against the importance of Polyploidy are: (a) that the large amount of gene duplication in polyploids dilutes the effects of new mutations so significant adaptive changes are unlikely, (b) that Polyploidy in animals is restricted mainly to asexual forms which are evolutionary dead ends, and (c) the number of polyploid animals relative to diploids is small.

Gynogenesis and Triploidy in the Viviparous Fish Poeciliopsis

Three all-female strains of the viviparous fish Poeciliopsis occur in the R�o Fuerte of Sinaloa, Mexico. Poeciliopsis lucida, a bisexual species, provides sperm for these monosexual forms which I designate as Cx, Cy, and Cz. Form Cy is a triploid that when test-mated to males of various species produces all-female, triploid offspring devoid of paternal characters. Both Cx and Cz are diploid and express characteristics of both parents.

Induction of hepatic tumors with 7,12-dimethylbenz(a)anthracene in two species of viviparous fishes (genus Poeciliopsis)

Liver neoplasms were induced in two species of viviparous fishes, Poeciliopsis lucida and P. monacha, by repeated short-term exposures to an aqueous suspension of 5 ppm 7,12-dimethylbenz[a]anthracene (DMBA). Predominantly hepatocellular neoplasms developed in 47 out of 106 fish surviving the period allowed for tumor development, 6–9 months after initial exposure to DMBA. The use of short-term exposure periods of 6 hr in young fish and up to 20 hr in adults reduced stress and mortalities caused by the toxicity of DMBA at 5 ppm. A lower concentration of DMBA, 0.25 ppm, failed to induce tumors in any of the surviving 230 of 295 young fish administered repeated 20-hr exposures. This study shows for the first time that DMBA is carcinogenic to fish.

Transplantable chemically-induced liver tumors in the viviparous fish Poeciliopsis

Transplantation of five liver tumors induced with the chemicals diethylnitrosamine (DEN) and 7,12-dimethylbenz[a]anthracene (DMBA) in small live-bearing fish of the genus Poeciliopsis is reported. Five permanent strains representing three distinct tumor types were established in isogenic hosts. Histological characterization of hepatocellular carcinoma, hemangiopericytic sarcoma, and cholangiocarcinoma and the development of the neoplasms in host fish is presented. Transplantability of the experimental liver tumors provides evidence of their malignant nature. Metastasis of the hepatocellular carcinoma occurred from tumor implants in the dorsal musculature or peritoneal cavity and from the hemangiopericytic sarcoma implanted in the intraperitoneal cavity.

Competition and Adaptation Among Diploid and Polyploid Clones of Unisexual Fishes

During the development of population biology as a science, genetic variation has been regarded as adaptive, maladaptive, or simply waiting to be drawn upon during times of environmental change. Attempts to evaluate the importance of variation, either in the production of heterosis or as adaptive polymorphisms in homogeneous, heterogeneous, or fluctuating environments, are generally confounded by two major problems: (1) identifying the forces of selection that are operational and (2) determining the phenotype (and associated genotype) upon which selection operates. In sexually reproducing organisms the effects of selection are continually obscured by recombination. Asexual organisms provide a simpler system for investigating the adaptive significance of genetic variation. Here, the genetic variation of a population is contained in the form of multiple genetic clones that, in the absence of recombination, magnify the effects of selection in successive generations.

Differences in courtship aggression among six clones of unisexual fish

Abstract Hybridization between the viviparous fishes Poeciliopsis monacha and P. lucida of northwestern Mexico has resulted in the formation of diploid and triploid all-female ‘species’, P. monacha-lucida and P. monacha -2 lucida . These females reproduce by mating back to P. lucida , and are essentially clonally reproducing sexual parasites superimposed on that species. In a series of behavioural experiments, one diploid clone proved to be significantly more aggressive than one triploid and four other diploid clones. No differences in aggression were exhibited among the other five clones. The aggressiveness of this one clone may explain why only two clones live in the small tributary where it is found but up to 10 diploid and triploid clones occur where it is absent.

Sexual selection among clones of unisexual fish (Poeciliopsis : Poeciliidae) : genetic factors and rare-female advantage

Interspecific hybridization between the viviparous fishes Poeciliopsis monacha and P. lucida has produced two all-female forms: P. monacha-lucida, which is diploid, and P. monacha-2 lucida, a triploid. To reproduce, both must be inseminated by males of P. lucida. Expansion of unisexual populations is limited by the ability to attract males, which discriminate against hybrid forms. Unisexual clones in nature differ in their proportions of pregnant females, and laboratory studies comparing reproductive behavior and pregnancy rates demonstrate that some clones are more successful than others at acquiring sperm. When clones are at equal frequencies in a population, some attract males more readily than others. Learning by males of P. lucida to discriminate against one unisexual clone does not generalize completely to new clones; thus, unfamiliar clones are more likely to be courted than familiar clones (Keegan-Rogers 1984). Although all members of a triploid clone are genetically identical, members of a diploid hemiclone share only an identical monacha genome. This study shows that the unfamiliar-female advantage persists among diploid hemiclones even when the lucida genome of clonal members varies, as in natural populations. Pregnancy experiments confirm that, at unequal clonal frequencies, individuals of a clone that is rare in a population and unfamiliar to a given male are more likely to be impregnated than those of a common or familiar clone. Males of all five strains of P. lucida used in this study (two highly inbred, one less inbred, and two relatively wild) preferred conspecific females to unisexuals. No differences were found among the strains in the strength of their tendencies to choose either conspecifics over unisexuals or unfamiliar clones over familiar clones. We discuss how genetic differences in sexual attractiveness among clones and a rare-female advantage may help to explain the widespread establishment and continued coexistence of numerous clones.

Assignment of six enzyme loci to multipoint linkage groups in Fishes of the genusPoeciliopsis (Poeciliidae): Designation of linkage groups III–V

Three new linkage groups of enzyme loci are described usingPoeciliopsis monacha × P. viriosa-derived interspecific backcross hybrids. Comparison to known linkage groups of the confamilial genusXiphophorus shows homology betweenXiphophorus linkage group I andPoeciliopsis linkage group III,Xiphophorus linkage group II andPoeciliopsis linkage group I, andXiphophorus linkage group IV andPoeciliopsis linkage group IV. Comparison of the gene content of other fish, amphibians, and mammal syntenic groups suggests retention of plesiomorphic vertebrate gene arrangements in at least two poeciliid linkage groups. Expansion of thePoeciliopsis gene map should be of utility in the identification of tumor regulatory genes through demonstration of linkage to biochemical markers.Three new linkage groups of enzyme loci are described usingPoeciliopsis monacha × P. viriosa-derived interspecific backcross hybrids. Comparison to known linkage groups of the confamilial genusXiphophorus shows homology betweenXiphophorus linkage group I andPoeciliopsis linkage group III,Xiphophorus linkage group II andPoeciliopsis linkage group I, andXiphophorus linkage group IV andPoeciliopsis linkage group IV. Comparison of the gene content of other fish, amphibians, and mammal syntenic groups suggests retention of plesiomorphic vertebrate gene arrangements in at least two poeciliid linkage groups. Expansion of thePoeciliopsis gene map should be of utility in the identification of tumor regulatory genes through demonstration of linkage to biochemical markers.

Heat induced liver cell proliferation in the livebearing fish Poeciliopsis

SynopsisLiver regeneration is induced by heat stress in the small viviparous fish, Poeciliopsis. Acute exposure to sublethal temperatures, one to two degrees below their killing temperature, damages tissue and initiates liver cell proliferation in P. lucida, P. monacha, and P. monacha-lucida hybrid clones, SYN-4 and SYN-5. Regeneration of liver cells began within 1–2 days following heat stress and proceeded over 5 days. Peak cell proliferation occurred 2–3 days after treatment in fish of all four genotypes. Cell proliferation was induced in the two all-female clones, SYN-4 and SYN-5, by exposure to 40.5° C for 60 minutes. This treatment imposed mortalities of 17.9% and 16.7%, respectively, whereas reduction of the temperature to 39.5°C and reduction of the time to 30 minutes resulted in no mortalities without significantly lowering the level of cell proliferation (p > 0.05). Liver cell proliferation induced by both heat treatments was significantly higher (p < 0.05) in the SYN-5 hybrids than in SYN-4. The induction of liver cell proliferation with sublethal temperature exposures is discussed as it may relate to chemical carcinogenesis in both feral and laboratory fish. Acute heat exposure may be used experimentally in fish as an independent stimulus for liver cell proliferation in carcinogenesis studies. In poikilothermic animals-heat exposure offers an alternative to surgical removal of approximately two-thirds of the liver, the method most frequently used in rodents to study the process of liver regeneration.