On the stability of dispensable constituents of the eukaryotic genome: stability of coding sequences versus truly hypervariable sequences in a clonal vertebrate, the amazon molly, Poecilia formosa.

Abstract

In clonal unisexual vertebrates, the genes specifying the males become dispensable. To study the fate of such genes the gynogenetic all-female fish Poeciiaformosa was treated with androgens. Phenotypic males were obtained that exhibited the complete set of male characteristics of closely related gonochoristic species, including body proportions, pigmentation, the extremely complex insemination apparatus of poeciliid fish, sexual behavior, and spermatogenesis. The apparent stability of such genic structures, including those involved in androgen regulation, is contrasted by high instability of noncoding sequences. Frequent mutations, their donal transmission, and at least two truly hypervariable loci leading to individual differences between these otherwise donal organisms were detected by DNA fingerprinting. These observations substantiate the concept that also in "ameiotic" vertebrates certain compartments of the genome are more prone to mutational alterations than others.Clonal organisms occur in natural populations of multicellular animals at all levels of organismic evolution, but within vertebrates they are confined to teleost fish, amphibians, and reptiles (1). They exist as unisexual lineages that exclude effective genetic recombination. In such animals the genetic information specifying the other sex has apparently become dispensable. These organisms are therefore uniquely suited to investigate questions centering on problems of "dispensable" genes (2). Moreover, the existence of naturally occurring clonal animals poses questions about their origin and abilities to evolve in the absence of meiotic recombination.The amazon molly, Poecilia formosa, is an all-female poeciliid fish that inhabits freshwater streams and brackish coastal lagoons over a broad geographic range from southeastern Texas to northeastern Mexico. The reproductive mechanism in P. formosa is gynogenesis, a modified form of parthenogenesis. Sperm do not participate in syngamy but are required to trigger the onset of embryogenesis in diploid eggs, containing only the maternal genetic information (3, 4). Except for rare cases (5) the offspring are clonal replicates of their mothers (6-8). In natural populations sperm is contributed by males of two closely related bisexual species that occur sympatric with P. formosa. In the northern range this is Poecilia latipinna (5), whereas in the mexican habitats it is Poecilia mexicana (10). Biochemical and cytological data (8) supported the initial hypothesis (11) that the amazon molly is a hybrid species between P. latipinna and P. mexicana. The question arose if the genes determining male sex, phenotype, and behavior are absent or not functional in P. formosa.Preliminary studies (12-15) reported that treatment with androgenic steroids induces several phenotypic changes that resembled the male characteristics of related species. Androgen-induced masculinization, therefore, appeared to be a useful approach to investigate if the genes instrumental in determining the male phenotype and behavio...

Unusual triploid males in a microchromosome-carrying clone of the Amazon molly, <i>Poecilia formosa</i>

“…By treatment with androgenic steroids during a sensitive period around birth, phenotypic males were obtained that exhibited the complete set of male secondary sex characters of the parental gonochoristic species, and developed ovotestis with all stages of sperm development including formation of the complex spermatozeugmata. For those morphological characters where the two parental species differ, an almost perfect intermediate phenotype was noticed (Schartl et al, 1991). Different degrees of "sex reversal" after androgen treatment were also observed by Haskins et al (1960), Hamaguchi and Egami (1980), and Turner and Steeves (1989).…”

Analysis of genetic heterogeneity among five gynogenetic clones of silver crucian carp, <i>Carassius auratus gibelio</i> Bloch, based on detection of RAPD molecular markers

“…Because this method does not require knowledge of specific sequences, but rather uses random 10-nucleotide primers, RAPD polymorphisms can be powerful tools for studying intraspecific genetic differentiation (Ferguson, 1995). In unisexual vertebrates reproduced by gynogenesis, clonal diversity has been discriminated by analysis of chromosomes (Sola et al, 1992(Sola et al, , 1993Zhu and Jiang, 1993), isozymes (Vrijenhoek, 1984;Zhu and Jiang, 1987;Avise et al, 1991), and mitochondrial DNA (Kraus and Miyamoto, 1990;Quattro et al, 1991Quattro et al, , 1992Hedges et al, 1992;Spolsky et al, 1992), as well as by DNA fingerprinting based on simple repeat oligonucleotide hybridization (Turner et al, 1990;Schartl et al, 1991Schartl et al, , 1995, but RAPD polymorphisms have not been applied to the study of gynogenetic animals. In this study, the RAPD technique was used to analyze genetic polymorphisms in different gynogenetic clones of the unique silver crucian carp, C. auratus gibelio Bloch.…”

“…These features place this particular silver crucian carp on an intermediate evolutionary step between uni-and bisexual reproduction in vertebrates. Recently, the paternal contribution to the clonal gynogens has been investigated at the organismal, cytogenetic, and molecular level in the Amazon molly (Schartl et al, 1991(Schartl et al, , 1995Schlupp et al, 1998). Because of their special advantages over other bisexual forms, all-female offspring of the silver crucian carp with paternal effects were propagated and used in aquaculture practice by heterologous sperm stimulation from the red common carp, Cyprinus carpio, since the 1980s (Jiang et al, 1983;Gui, 1997).…”

Evaluation of the carcinogenic effects of furazolidone and its metabolites in two fish species

“…11,12 Poecilia formosa is a female lineage of live bearing fish that undergo frequent mutations, possess clonal transmission and at least two truly hypervariable loci that substantiate the concept that mollies are prone to mutational alterations both from physical and chemical stimuli. [13][14][15] In fact, this fish species has been regarded as more advantageous than the use of rodents to test potential carcinogens. 15 Hence, results in this trial support the use of this fish species for the screening of potentially carcinogenic substances.…”

“…7 An ornamental gynogenetic fish species, Poecilia formosa (mollies) whose progenitors and descendants are genetically identical, is known to be prone to develop tumors, in part due to their peculiar breeding process. 8 This species has been regarded as a suitable model to test the carcinogenic potential of drugs, [9][10][11][12][13][14][15] and may be utilized to study the direct carcinogenic effects of FZD, as well as the indirect effects of AOZ. Also, a comparison of such effects was made in tilapia (Oreochromis niloticus).…”