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Biomedical subjects

F J Ayala

Publications and source records attributed to F J Ayala.

At least 19 recordsLinked to original sources
PubMed1992

Structure and sequence of the Cu,Zn Sod gene in the Mediterranean fruit fly, Ceratitis capitata: intron insertion/deletion and evolution of the gene.

We have cloned a 4-kb region encompassing the Cu,Zn superoxide dismutase (Sod) gene from a genomic library of the Mediterranean fruit fly, Ceratitis capitata, using a cDNA probe from Drosophila melanogaster. The coding sequence of 462 bases is equally as long as that in Drosophila species. The rate of amino acid replacement over the past 100 million years is approximately the same in the Diptera and in mammals, thus excluding the hypothesis (proposed to account for an apparent acceleration in rate of evolution of Sod over geological time) that the evolution of the SOD protein is much higher in the mammals than in other organisms. The coding region is interrupted by two introns in Ceratitis, whereas only one occurs in Drosophila. Phylogenetic comparisons indicate that the second intron was present in the common dipteran ancestor, but was lost shortly after the divergence of the Drosophila lineage from other Diptera. Analysis of the exon/intron structure of Sod in various animal phyla, plants, and fungi indicates that intron insertions as well as deletions have occurred in the evolution of the Sod gene.

Amino Acid Sequence↗
PubMed1992

Phylogenetic reconstruction of the Drosophila obscura group, on the basis of mitochondrial DNA.

We have constructed restriction-site maps of the mtDNAs in 13 species and one subspecies of the Drosophila obscura group. The traditional division of this group into two subgroups (affinis and obscura) does not correspond to the phylogeny of the group, which shows two well-defined clusters (the Nearctic affinis and pseudoobscura subgroups) plus a very heterogeneous set of anciently diverged species (the Palearctic obscura subgroup). The mtDNA of Drosophila exhibits a tendency to evolve toward high A+T values. This leads to a "saturation" effect that (1) begets an apparent decrease in the rate of evolution as the time since the divergence of taxa increases and (2) reduces the value that mtDNA restriction analysis has for the phylogenetic reconstruction of Drosophila species that are not closely related.

Animals↗
PubMed1992

Structure and sequence of the Cu, Zn superoxide dismutase gene of Chymomyza amoena: phylogeny of the genus and codon-use evolution.

We have cloned and sequenced the Cu, Zn superoxide dismutase gene of Chymomyza amoena. The coding sequence has the same length as in Drosophila species and in Ceratitis capitata. There are two introns, located at the same sites as in Ceratitis. The second intron is absent in Drosophila: this places Chymomyza outside the Drosophila lineage, contrary to proposals based on anatomical and other evidence. The nucleotide or amino acid distances support a phylogeny in which Ceratitis first branches off the common stem, then Chymomyza splits before the divergence of the two major Drosophila subgenera. The estimated divergence times are 58 million for Chymomyza-Drosophila; 48 million years for the Drosophila subgenera. During the intervening 10 million years, the Drosophila lineage lost the second intron and evolved distinct codon-preferences: the G + C use in the third coding positions is increased by 69% in Drosophila relative to Chymomyza or Ceratitis.

Amino Acid Sequence↗
PubMed1991

Evolution of behavior by density-dependent natural selection.

Theories of density-dependent natural selection predict that evolution should favor those genotypes with the highest per capita rates of population growth under the current density conditions. These theories are silent about the mechanisms that may give rise to these increases in density-dependent growth rates. We have observed the evolution of six populations of Drosophila melanogaster recently placed in crowded environments after nearly 200 generations at low-population density in the laboratory. After 25 generations in these crowded cultures all six populations showed the predicted increase in population growth rates at high-population density with the concomitant decrease in their growth rates at low densities. These changes in rates of population growth are accompanied by changes in the feeding and pupation behavior of the larvae: those populations that have evolved at high-population densities have higher feeding rates and are less likely to pupate on or near the food surface than populations maintained at low densities. These changes in behavior serve to increase the competitive ability of larvae for limited food and reduce mortality under crowded conditions during the pupal stage of development. A detailed understanding of the mechanisms by which populations evolve under density-dependent natural selection will provide a framework for understanding the nature of trade-offs in life history evolution.

Animals↗
PubMed1991

Density-dependent natural selection and trade-offs in life history traits.

Theories of density-dependent natural selection state that at extreme population densities evolution produces alternative life histories due to trade-offs. The trade-offs are presumed to arise because those genotypes with highest fitness at high population densities will not also have high fitness at low density and vice-versa. These predictions were tested by taking samples from six populations of Drosophila melanogaster kept at low population densities (r-populations) for nearly 200 generations and placing them in crowded cultures (K-populations). After 25 generations in the crowded cultures, the derived K-populations showed growth rate and productivity that at high densities were elevated relative to the controls, but at low density were depressed.

Animals↗
PubMed1991

Are eukaryotic microorganisms clonal or sexual? A population genetics vantage.

We argue that the mode of reproduction of microorganisms in nature can only be decided by population genetic information. The evidence available indicates that many parasitic protozoa and unicellular fungi have clonal rather than sexual population structures, which has major consequences for medical research and practice. Plasmodium falciparum, the agent of malaria, is a special case: the scarce evidence available is contradictory, some suggesting that uniparental lineages may exist in nature. This is puzzling (because P. falciparum is known to have a sexual stage) and poses a challenge that can be readily settled by ascertaining the frequency distribution of genotypes in natural populations.

Animals↗
PubMed1991

Clonal defence.

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Animals↗
PubMed1991

Fertility and viability at the Sod locus in Drosophila melanogaster: non-additive and asymmetric selection.

Experiments were designed to test in Drosophila melanogaster the effect of mating type at the Sod locus on fertility and viability. The experiments show that fertility is neither additive (or multiplicative) nor symmetric, i.e. that the fertility of a mating type cannot be predicted from the average fertility of the two genotypes involved in the mating. There is no significant male x female interaction with respect or progeny viability; but the interaction is significant for productivity, i.e. when fertility and viability are jointly taken into account. There is overdominance with respect to female fertility, but not with respect to male fertility or to viability. There also is alloprocoptic selection with respect to fertility and with respect to productivity, i.e. mating between like homozygotes are less fertile and productive than matings between dissimilar homozygotes. Selection at the Sod locus yields stable polymorphic equilibria, with the frequency of the F allele predicted at P = 0.641 or 0.695, respectively for low and high larval density.

Animals↗
PubMed1991

Two modes of balancing selection in Drosophila melanogaster: overcompensation and overdominance.

Overdominance is often invoked to account for the extensive polymorphisms found in natural populations of organisms; overcompensation, however, may be equally or more important. Overcompensation occurs when limiting resources are better exploited by a genetically mixed than by a uniform population, and is often causally related to frequency-dependent selection. We have designed experiments to test whether overcompensation occurs in Drosophila melanogaster, using the Sod locus as a marker. Tests are made at each of two densities and two temperatures for cultures with desired genetic compositions. Both temperature and density have statistically significant effects on the per-female productivity of the cultures. More important, there are strong effects due to overcompensation. Cultures that are more polymorphic are also more productive than less polymorphic ones even when the level of individual heterozygosity is the same in all. There is also overdominance for the Sod locus: the heterozygotes are more productive than either homozygote at every temperature and density, and the differences are statistically significant in several cases. These results corroborate previous studies showing that overdominance may contribute to the maintenance of the Sod polymorphisms. Moreover, our results indicate that the significance of overcompensation as a mechanism to account for polymorphism in natural populations deserves further investigation.

Analysis of Variance↗
PubMed1991

The rate of Cu,Zn superoxide dismutase evolution.

The rate of amino acid replacement in Cu,Zn SOD greatly departs from the expectations of the molecular clock. We examine 27 Cu,Zn SOD sequences available and conclude that: (1) the SOD enzymes from different mammal families differ from each other by roughly the same number of replacements, which is consistent with a simultaneous mammalian radiation; (2) over the most recent 60 million years (MY) the rate of SOD evolution is fairly high (15 aa/100 aa/100 MYR) and may be considered constant; (3) the rate of accumulation of amino acid replacements since the divergence of fungi, plants and animals to the present is inconstant along different branches of the evolutionary tree; moreover it steadily decreases with time, to the same extent in all lineages; (4) some comparisons exhibit divergences that are in any case greater than expected from a Poisson process on the assumption of a molecular clock; (5) plant chloroplast enzymes display fewer differences from each other than cytoplasmic ones; (6) bacteriocuprein (from Photobacterium leiognathi), fluke and human extracellular SOD are all three extremely remotely related to one another and to the SOD of other eukaryotes. The process of consistent decline of the rate of evolution of Cu, Zn SOD can be described by a number of mathematical functions. We explore simple models that assume constant rates and might be applicable to other proteins or genes that apparently evolve at disparate rates.

Amino Acid Sequence↗
PubMed1991

A truncated P element is inserted in the transcribed region of the Cu,Zn SOD gene of an SOD "null" strain of Drosophila melanogaster.

The decreased Cu,Zn SOD activity (less than 5%) in a "null" SODCA1 Drosophila melanogaster strain isolated in our laboratory is due to the insertion of a truncated P element into the transcribed region of the Cu,Zn SOD gene. Using a cDNA Cu,Zn SOD probe from a wild type D. melanogaster (F allele) we isolated an EcoRI Cu,Zn SOD clone from an EMBL3 genomic library of the SODCA1 strain, subcloned it, restriction-mapped and partially sequenced it. The 2.5 kb clone consists of a wild-type 1.84 kb EcoRI fragment containing the Cu,Zn SOD gene previously isolated in our laboratory, with an insertion of 0.68 kb derived (by an internal deletion) from an autonomous, 2.9 kb P element. The insertion starts 21 bp upstream from the coding sequence and causes an 8 bp target site duplication characteristic of P elements. A point mutation in the second exon results in a replacement of Asn by Lys at position 96, confirming that the mature protein encoded by the SOCCA1 is the same one encoded by the S allele, commonly found in natural populations. The diminished expression of SODCA1 allele is most possibly due to a reduction of the rate of transcription attributable to the insertion of the P element.

Alleles↗
PubMed1990

Overcompensation as a mechanism for maintaining polymorphism: egg-to-adult viability in Drosophila.

Frequency-dependent selection may be accounted for, in ecological terms, by the differential effectiveness of alternative genotypes in exploiting limiting environmental resources. Differentiation in resource exploitation among genotypes implies in turn that a mix of genotypes may exploit more fully the resources than a genetically uniform population, a phenomenon called 'overcompensation'. Experiments designed to test for overcompensation show that highly polymorphic populations can support larger numbers of individuals per food unit than less polymorphic populations. This difference cannot be attributed to the level of individual heterozygosity, which is the same in both types of populations.

Animals↗
PubMed1990

A clonal theory of parasitic protozoa: the population structures of Entamoeba, Giardia, Leishmania, Naegleria, Plasmodium, Trichomonas, and Trypanosoma and their medical and taxonomical consequences.

We propose a general theory of clonal reproduction for parasitic protozoa, which has important medical and biological consequences. Many parasitic protozoa have been assumed to reproduce sexually, because of diploidy and occasional sexuality in the laboratory. However, a population genetic analysis of extensive data on biochemical polymorphisms indicates that the two fundamental consequences of sexual reproduction (i.e., segregation and recombination) are apparently rare or absent in natural populations of the parasitic protozoa. Moreover, the clones recorded appear to be stable over large geographical areas and long periods of time. A clonal population structure demands that the medical attributes of clones be separately characterized; ubiquitous clones call for priority characterization. Uniparental reproduction renders unsatisfactory Linnean taxonomy; this needs to be supplemented by the "natural clone" as an additional taxonomic unit, which is best defined by means of genetic markers.

Animals↗