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PubMed2005

Evolution of sexuality: biology and behavior.

Sexual reproduction in animals and plants is far more prevalent than asexual reproduction, and there is no dearth of hypotheses attempting to explain why. Even bacteria and viruses, which reproduce by cloning, engage in promiscuous horizontal gene exchange ("parasexual reproduction") on such short time scales that they evolve genotypic diversity even more rapidly than eukaryotes. (We confront this daily in the form of antimicrobial resistance.) The host-parasite and host-pathogen arms race purports to explain the prevalence of sexual reproduction, yet there are over a dozen other hypotheses, including the proposition that sexual reproduction purges the genome of deleterious mutations. An equally daunting challenge is to understand, in terms of evolutionary logic, the jungle of diverse courtship and mating strategies that we find in nature. The phenotypic plasticity of sex determination in animals suggests that the central nervous system and reproductive tract may not reach the same endpoint on the continuum between our stereotypic male and female extremes. Why are there only two kinds of gametes in most eukaryotes? Why are most flowering plants, and few animals, hermaphroditic? Why do male animals compete more for access to females than the other way around in most animals that have been studied?This review presents more questions than answers, but an extraordinary wealth of data has been collected, and new genetic techniques will provide new answers. The possible relevance of these data to human sexuality will be discussed in a future article.

Journal Article↗
PubMed2000

High levels of sequence polymorphism and linkage disequilibrium at the telomere of 12q: implications for telomere biology and human evolution.

The human Xp/Yp telomere-junction region exhibits high levels of sequence polymorphism and linkage disequilibrium. To determine whether this is a general feature of human telomeres, we have undertaken sequence analysis at the 12q telomere and have extended the analysis at Xp/Yp. A total of 22 single-nucleotide polymorphisms (SNPs) and one 30-bp duplication were detected in the 1,870 bp adjacent to the 12q telomere. Twenty polymorphic positions were in almost complete linkage disequilibrium, creating three common diverged haplotypes accounting for 80% of 12q telomeres in the white population. A further 6% of 12q telomeres contained a 1,439-bp deletion in the DNA flanking the telomere. The remaining 13% of 12q telomeres did not amplify with the primers used (nulls). The distribution of telomere (TTAGGG) and variant repeats within 12q telomeres was hypervariable, but alleles with similar distribution patterns were associated with the same haplotype in the telomere-adjacent DNA. These data suggest that 12q telomeres, like Xp/Yp telomeres, exhibit low levels of homologous recombination and evolve along haploid lineages. In contrast, high levels of homologous recombination occur in the adjacent proterminal regions of human chromosomes. This suggests that there is a localized telomere-mediated suppression of recombination. In addition, the genetic characteristics of these regions may provide a source of deep lineages for the study of early human evolution, unaffected by both natural selection and recombination. To explain the presence of a few diverged haplotypes adjacent to the Xp/Yp and 12q telomeres, we propose a model that involves the hybridization of two archaic hominoid lineages ultimately giving rise to modern Homo sapiens.

Base Sequence↗
PubMed1981

The problem of missing links: today and yesterday.

The problems of "missing links" over the years since Darwin's On the Origin of Species is analyzed in the perspective of our increased knowledge of the fossil record and our changing concepts of evolution, geology, and biology. The history of application of evolutionary theory to the fossil record and the consequences of the conflict of categorical, essential classification and evolutionary continuity are examined, as they relate to the "naive" idea of "missing links". Transitions between taxonomic categories are treated at two levels: the infraspecies-species level, and the supraspecies level. In the latter category case analyses of transitions within and between the major groups of metazoans and metaphytes are made, with special attention to the evidence of origins of groups in the fossil record. Throughout, the impacts on evolutionary theory of changes in knowledge and interpretations of the record of the history of life are considered. The problem of the existence of linkages and phylogenies at the species and generic levels has been much reduced during the last one hundred and twenty years. How this reduction supports or denies Darwin's concepts of phyletic gradualism is still a matter of interpretation of the evidence. At familial and higher levels, the establishment of linkages between categories has been much less successful, and decreasingly so at each successive higher level. Under the very best circumstances, however, morphological and stratigraphically graded transitions between classes and subclasses have been found. At the level of phyla and higher categories, any information on transitions as far as the fossil record is concerned is essentially non-existent. Fairly standard patterns of transitions between high categories can be established on the basis of the optimal cases, and these point up the continuing problems in evolutionary theory as being the interrelationships and integration of micro-evolutionary and macro-evolutionary processes.

Amphibians↗
PubMedDate not supplied

Chromosomal and protein evolution in morphologically similar species of Praomys sensu lato (Rodentia, Muridae).

Evidence of extensive chromosomal evolution in a biologically and economically important group of African murids of the Praomys/Mastomys complex was provided by examination of G- and C-band chromosomal data on P. coucha (2n = 32), P. fumatus (2n = 38), P. hildebrandti (2n = 32), P. jacksoni (2n = 28), P. misonnei (2n = 36), and P. cf. tullbergi (2n = 35). A coding system was developed for the chromosomal characters, and analyses were performed by a computer program to find the shortest tree with a minimum of 35 autosomal rearrangements (pericentric inversions, complex translocations, centric fusions, centric fissions, tandem fusions, euchromatic additions, and heterochromatic additions). The resulting phylogenetic hypothesis differs from traditionally accepted hypotheses regarding this complex group of rodents. The cytogenetic data show that 1) there is no support for the dichotomy of Mastomys/Praomys previously based on morphology, 2) the 2n = 32 species from eastern Africa (P. hildebrandti) is distinct from the 2n = 32 species from southern Africa (P. natalensis), and 3) there is a close association between P. jacksoni and P. cf. tullbergi. Polyacrylamide gel electrophoresis of liver membrane proteins demonstrated few differences in protein mobilities between species and even fewer between individuals of the same species taken from different habitats and localities in Kenya. Monoclonal antibodies produced against liver proteins of one species and tested for reactivity to other species confirmed the evolutionary similarity of species of this complex. This immunologic approach may provide a robust data set for future phylogenetic studies of muroid rodents.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2000

The fitness effects of spontaneous mutations in Caenorhabditis elegans.

Spontaneous mutation to mildly deleterious alleles has emerged as a potentially unifying component of a variety of observations in evolutionary genetics and molecular evolution. However, the biological significance of hypotheses based on mildly deleterious mutation depends critically on the rate at which new mutations arise and on their average effects. A long-term mutation-accumulation experiment with replicate lines of the nematode Caenorhabditis elegans maintained by single-progeny descent indicates that recurrent spontaneous mutation causes approximately 0.1% decline in fitness per generation, which is about an order of magnitude less than that suggested by previous studies with Drosophila. Two rather different approaches, Bateman-Mukai and maximum likelihood, suggest that this observation, along with the observed rate of increase in the variance of fitness among lines, is consistent with a genomic deleterious mutation rate for fitness of approximately 0.03 per generation and with an average homozygous effect of approximately 12%. The distribution of mutational effects for fitness appears to have a relatively low coefficient of variation, being no more extreme than expected for a negative exponential, and for one composite fitness measure (total progeny production) approaches constancy of effects. These results are derived from assays in a benign environment. At stressful temperatures, estimates of the genomic deleterious mutation rate (for genes expressed at such temperatures) is sixfold lower, whereas those for the average homozygous effect is approximately eightfold higher. Our results are reasonably compatible with existing estimates for flies, when one considers the differences between these species in the number of germ-line cell divisions per generation and the magnitude of transposable element activity.

Animals↗
PubMed2001

Hybrid populations selectively filter gene introgression between species.

Hybrids have long been recognized as a potential pathway for gene flow between species that can have important consequences for evolution and conservation biology. However, few studies have demonstrated that genes from one species can introgress or invade another species over a broad geographic area. Using 35 genetically mapped restriction fragment length polymorphism (RFLP) markers of two species of cottonwoods (Populus fremontii x P. angustifolia) and their hybrids (n = 550 trees), we showed that the majority of the genome is prohibited from introgressing from one species into the other. However, this barrier was not absolute; Fremont cpDNA and mtDNA were found throughout the geographic range of narrowleaf cottonwood, and 20% of the nuclear markers of Fremont cottonwood introgressed varying distances (some over 100 km) into the recipient species' range. Rates of nuclear introgression were variable, but two nuclear markers introgressed as fast as the haploid, cytoplasmically inherited chloroplast and mitochondrial markers. Our genome-wide analysis provides evidence for positive, negative, and neutral effects of introgression. For example, we predict that DNA fragments that introgress through several generations of backcrossing will be small, because small fragments are less likely to contain deleterious genes. These results argue that recombination will be important, that introgression can be very selective, and that evolutionary forces within the hybrid population to effectively "filter" gene flow between species. A strong filter may make introgression adaptive, prevent genetic assimilation, lead to relaxed isolating mechanisms, and contribute to the stability of hybrid zones. Thus, rather than hybridization being a negative factor as is commonly argued, natural hybridization between native species may provide important genetic variation that impacts both ecological and evolutionary processes. Finally, we propose two hypotheses that contrast the likelihood of contemporary versus ancient introgression in this system.

Biological Evolution↗
PubMed1967

Sickle-cell trait in human biological and cultural evolution. Development of agriculture causing increased malaria is bound to gene-pool changes causing malaria reduction.

The particular agricultural adaptation we have been considering is the ultimate determinant of the presence of malaria parasites in the intracellular environment of the human red blood cell. This change in the cellular environment is deleterious for normal individuals, but individuals with the sickle-cell gene are capable of changing their red-cell environment so that intense parasitism never develops. Normal individuals suffer higher mortality rates and lower fertility rates in a malarious environment than individuals with the sickle-cell trait do, so the latter contribute proportionately more people to succeeding generations.

Africa, Eastern↗
PubMed1998

Phylogeny and evolution of host-parasitoid interactions in hymenoptera.

Recent studies of hymenopteran phylogeny using both comparative morphology and DNA sequence data have greatly enhanced our understanding of the evolution of that order. Resulting phylogenetic hypotheses make possible more rigorous investigations of the evolution of various biological life-styles, among them the parasitoid habit. This paper reviews the current findings from higher-taxon phylogenetic analyses of the order. A "consensus" phylogeny derived from these findings is used to trace the most likely evolutionary pathways leading to the current diversity of parasitoid habits. Taxa and biological phenomena for which our current understanding is fragmentary are highlighted. Based on current evidence, it appears that parasitism arose, from mycophagous ancestors, a single time within the order. Many subsequent elaborations of the parasitic mode of life (e.g. endoparasitism, secondary phytophagy, etc) apparently evolved independently more than once.

Animals↗
PubMed1996

Modeling marrow damage from response data: evolution from radiation biology to benzene toxicity.

Consensus principles from radiation biology were used to describe a generic set of nonlinear, first-order differential equations for modeling toxicity-induced compensatory cell kinetics in terms of sublethal injury, repair, direct killing, killing of cells with unrepaired sublethal injury, and repopulation. This cellular model was linked to a probit model of hematopoietic mortality that describes death from infection and/or hemorrhage between 5 and 30 days. Mortality data from 27 experiments with 851 dose-response groups, in which doses were protracted by rate and/or fractionation, were used to simultaneously estimate all rate constants by maximum-likelihood methods. Data used represented 18,940 test animals: 12,827 mice, 2925 rats, 1676 sheep, 829 swine, 479 dogs, and 204 burros. Although a long-term, repopulating hematopoietic stem cell is ancestral to all lineages needed to restore normal homeostasis, the dose-response data from the protracted irradiations indicate clearly that the particular lineage that is critical to hematopoietic recovery does not resemble stemlike cells with regard to radiosensitivity and repopulation rates. Instead, the weakest link in the chain of hematopoiesis was found to have an intrinsic radioresistance equal to or greater than stromal cells and to repopulate at the same rates. Model validation has been achieved by predicting the LD50 and/or fractional group mortality in 38 protracted-dose experiments (rats and mice) that were not used in fitting of model coefficients.

Animals↗
PubMedDate not supplied

[Morphologic characterization and biological aspects of evolutive forms of Babesia bigemina (Smith and Kilborne, 1893) (Protozoa: Babesiidae) in Boophilus microplus (Canestrini, 1887)].

The development of Babesia bigemina in Boophilus microplus were studied in experimental conditions, using crossed-breed bovine from free-area of these parasites. Stages of the hemoparasites were observed in the tick vector, starting from the infected red-blood cells observed in the gut of engorged females, from the first 24 hours after detachment to the emergence of sporokinets in the larvas. In the period from 24 to 48 hours after detachment of the engorged females (DEF), the presence of some infected red-blood cells was verified, beside the occurrence of ray bodies and of vermiculars forms, known as oocynets. Since 72 hours after the DEF, the of okinets presence was observed in the cytoplasm of the epithelium cells besides of great sporokinets number in development. At same period, the presence of sporokinets of B. bigemina in the hemolymph samples was observed inside the hemocytes. After the fourth day of incubation beside the presence of the sporokinets was also verified in the Malpighi's tubes and ovaries. As well as in the ticks eggs from the sporokinets were also observed ticks eggs from the fourty day after the natural detachment of the engorged females of the host.

Animals↗