Asexual Origin Research Articles

Molecular characterization of cyclic and obligate parthenogens in the aphid Rhopalosiphum padi (L.).

Holocyclic clones of the aphid Rhopalosiphum padi (L.) reproduce by cyclic parthenogenesis, whereas anholocyclic individuals are obligate parthenogens. Mitochondrial DNA and (mtDNA) and random amplified polymorphic DNA markers in R. padi as well as plasmid DNA markers of its bacterial endosymbiont, Buchnera aphidicola, were examined to determine the extent of genetic divergence between clones with these differing breeding systems. These analyses revealed that cyclically parthenogenetic lineages possessed differing mtDNA and plasmid haplotypes than most obligately asexual clones. The extent of sequence divergence between the maternally inherited molecules suggest a relatively ancient origin of asexuality. Our work also identified a random amplified polymorphic DNA marker linked to the life-cycle variation in R. padi. This marker not only permits the rapid diagnosis of breeding system but sets the stage for studies to identify the gene(s) controlling this variation in mode of reproduction.

POLYPHYLETIC ORIGINS OF ASEXUALITY INDAPHNIA PULEX. II. MITOCHONDRIAL-DNA VARIATION

Allozyme studies of the cladoceran Daphnia pulex have shown that most populations reproduce by obligate parthenogenesis, although some cyclically parthenogenetic populations remain throughout the southern portion of its range. Clonal diversity within the obligate parthenogens is extremely high and has been attributed to the polyphyletic origin of asexuality. Specifically, it has been proposed that the clonal diversity in the obligate parthenogens was generated via the spread of a sex-limited meiosis suppressor through populations of a cyclically parthenogenetic ancestor. In this study, analysis of polymorphism of restriction-endonuclease sites in the mitochondrial genome, in conjunction with allozyme analysis, was used to determine whether obligate parthenogenesis has a monophyletic or polyphyletic origin in D. pulex. An allozyme survey of 77 populations from Ontario and Michigan was first conducted to determine breeding systems and levels of clonal diversity (Hebert et al., 1989). Mitochondrial-DNA variation was then surveyed in one isolate of each clone from each population reproducing by obligate parthenogenesis and in 2-4 isolates from each population reproducing by cyclic parthenogenesis. Seventeen restriction enzymes were used in this analysis. Thirty-five mitochondrial genotypes were found among the 36 obligate clones (as identified by allozyme analysis), while 17 mitochondrial genotypes were identified among 40 cyclic isolates from 14 populations. Five mitochondrial genotypes were found in both groups. Parsimony and phenetic-clustering methods were used to construct trees showing the genetic relationship among mitochondrial genotypes. The results clearly show that obligate parthenogenesis had a polyphyletic origin in this species. The close relationship between cyclic and obligate parthenogens in the Great Lakes region suggests that many obligate clones have recently been derived from cyclic populations and that the generation of clones is still occurring in this area. Patterns of clonal diversity based on the joint consideration of allozyme and mitochondrial-DNA data are discussed.

Considerations of life history, morphology and taxonomy inRhodophysema georgiiBatters (Rhodophyta, Palmariales)

The life history of Rhodophysema georgii from the Atlantic coast of Canada is shown to be exclusively asexual, with direct development of tetrasporophytes from mitotically derived tetraspores. Sexual structures are completely absent and the presence of intercalary sporangial stalk cells provides further morphological evidence of the asexual origin of the tetrasporangia. Chromosome numbers of 17 or 18 were found both in vegetative cells and tetrasporocytes. The occurrence of a large-celled medulla, which characterizes globular thalli of R. georgii, is variable within the species, and the mechanism of its formation, by cellular fusion, is not limited to this species within the genus Rhodophysema. Thus, R. georgii is not fundamentally different in vegetative ontogeny and morphology from other, exclusively crustose, species of Rhodophysema. The morphological features related to life history and medullary cell formation were also observed in lectotype material of R. georgii, thus confirming the identity of the Canadian material.

Somatic selection in fungi.

IN a recent communication, Calpouzos1 reported that prolonged selection of sporulating material from recent single or mass spore isolates of the fungus Cercospora musae gradually increased the intensity and uniformity of sporulation in later cultures even though the spores selected were always of asexual origin. He suggested that if the original isolates had been heterokaryotic this observation was in agreement with the gradual sorting out, by selection, of a nuclear type having a greater spore-producing potential. Although this is a possibility in the case reported by Calpouzos, a similar situation has been encountered in my own experiments under conditions where heterokaryosis is ruled out as a possible explanation.

On the Asexual Origin of the Ciliated Sponge Larva

On the Asexual Origin of the Ciliated Sponge Larva