EVIDENCE FOR A MOSAIC HYBRID ZONE IN THE GRASS SHRIMP PALAEMONETES KADIAKENSIS (PALAEMONIDAE) AS REVEALED BY MULTIPLE GENETIC MARKERS

Molecular techniques provide powerful tools for studying the geographic structure of hybrid zones and the dynamics of gene exchange between incipient species. We examined allozyme variation at five loci (PGM, GPI, MDH-1, MDH-2, and LDH) for 27 populations of Palaemonetes kadiakensis from the central, coastal, and eastern regions of Texas. Central Texas populations of P. kadiakensis exhibited highly significant linkage disequilibrium and departures from Hardy-Weinberg genotype proportions. In populations with linkage disequilibrium, allelic differences at GPI defined two types of P. kadiakensis, designated A and B. Both types existed in central Texas with little or no evidence of interbreeding, whereas the populations from all other localities showed complete introgression of type B alleles into the type A gene pool. We also examined ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA) variation in a subset of populations, chosen to cover a range of geographic locations and levels of linkage disequilibrium. Two groups of mtDNA haplotypes and two restriction fragment patterns for the rDNA corresponded to allozyme type A and B individuals in populations exhibiting linkage disequilibrium. In populations with ongoing hybridization, all hybrid animals (N= 15) exhibited type A mtDNA. Exhibition of type A mtDNA indicated that type A females had mated successfully with type B males, but type B females had not mated successfully with type A males. Genotype distributions suggest reduced reproduction by hybrid offspring in central Texas populations. These patterns are consistent with a mosaic model of hybrid zone dynamics.     

Effect of selection intensity and population size on percent oil in maize,Zea mays L

Summary The effect of selection intensity and population size on the response to selection for percent oil in the grain of maize (Zea mays L.) was evaluated in a replicated experiment over ten cycles of selection. An open-pollinated variety, Armel's Reid Yellow Dent, was divided into subpopulations of 6,10 and 50 plants. Selection proportions of 17% and 5% were imposed upon each subpopulation. Selection was based on the percentage of oil in individual kernels as determined by wide-line nuclear magnetic resonance spectroscopy. As expected, total response to selection increased with larger population sizes and selection intensities. The concave shape of the response curves suggested that an appreciable part of the genetic variance can be attributed to additive genes at high initial frequencies, dominance genes at low initial frequencies, or to the generation of negative linkage disequilibrium due to selection. The consistently greater loss of vigor experienced by the more intensely selected populations reflects the enhancement of inbreeding due to artificial selection, an effect that increases with the intensity of selection. The results indicate that combined selection, based on kernels and using within- and amongfamily information, will be more efficient than other conventional selection procedures, including the normal combined scheme where selection is based on plants.Deceased     

Variation in alpha-L-fucosidase properties among 28 inbred mouse strains: Six strains have high enzyme activity and heat-stabile enzyme with a variant pH-activity curve; twenty-two strains have low activity and heat-labile enzyme

Alpha-L-fucosidase in tissues of 28 inbred mouse strains varied with respect to three properties: high or low heat stability, a pH-activity curve with high or low relative activity at pH 2.8, and high or low activity. Alpha-L-fucosidase from six strains (A/J, BDP/J, LP/J, P/J, SEA/GNJ, and 129/J) had high heat stability, high pH 2.8 relative activity, and high activity, whereas the other 22 strains all had low heat stability, low pH 2.8 relative activity, and low activity. The heat-stability difference was seen in all organs tested (brain, liver, kidney, spleen, heart, skeletal muscle, lung, and testis) for two heat-stabile strains (P/J and 129/J) and four heat-labile strains (C57 BL/6J, C3H/HeJ, DBA/2J, and BALB/cJ) studied in detail. The findings suggested that two structural variants of alpha-L-fucosidase, probably genetically determined, exist in these 28 inbred mouse strains, although the presence of linkage disequilibrium between alleles of tightly linked structural and regulatory genes could not be excluded.This work was supported by grants from the National Institutes of Health (NS-15281 and NS-11766), the Muscular Dystrophy Association (H. Houston Merritt Clinical Center for Muscular Dystrophy and Related Diseases), the March of Dimes Birth Defects Foundation, and a generous gift from the Alexander Rapaport Foundation.     

ASSORTATIVE MATING AND NATURAL SELECTION IN AN IRIS HYBRID ZONE

The phenology of different genotypes and the distribution of genetic variation among flowering plants and their progeny were examined to assess the levels of assortative mating and selection in a hybrid population of Iris. This study and a previous survey of RAPD nuclear markers and chloroplast markers indicate that the population consists of parental genotypes and recombinant hybrid genotypes that are similar to the parental species (I. fulva and I. brevicaulis), although lacking intermediate genotypes. Early in the season only I. fulva genotypes produced flowers, but as flowering in these plants decreased, the hybrid genotypes and I. brevicaulis genotypes began flowering, resulting in a 24-d period of coincidental flowering. The genotypic distribution of seeds produced during the period of flowering overlap contained a high frequency of intermediate genotypes that were not present in the adult generation. The degree of effective assortative mating was examined by comparing the observed progeny genotypic distributions with expected distributions from a mixed-mating model. The model included selfing and random outcrossing to the nearest plants that had pollen-bearing flowers on the day the recipient flower was receptive. The observed genotypic distribution of progeny from plants with I. brevicaulis chloroplast DNA (cpDNA) was not significantly different from the expected distribution. For I. fulva genotypes, however, there were higher than expected frequencies in the extreme genotypic classes, although intermediate genotypes were absent, indicating that these plants were preferentially mating with similar genotypes. Compared with the extreme genotypes, a larger proportion of the intermediate seed progeny produced were aborted, indicating that intermediate genotypes have lower viability. On the basis of the observed progeny genotypes and genetic disequilibria estimates for the adults and the progeny, there appears to be a pattern of effective asymmetrical mating in this population. This asymmetry is most likely due to pollen-style interactions that reduce the fertilization ability of genetically dissimilar pollen, or preferential abortion of genetically intermediate zygotes by I. fulva-like genotypes. The lack of any apparent discrimination by I. brevicaulis-like genotypes creates a directional exchange of nuclear genetic elements that will have implications for introgression and the evolution of hybrid genotypes.     

SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS BY THREE SPECIES OF MARINE DIATOM1

The utilization of inorganic carbon by three species of marine diatom, Skeletonema costatum (Grev.) Cleve. Ditylum brightwellii (West) Grun., and Chaetoceros calcitrans Paulsen was investigated using an inorganic carbon isotopic disequilibnum technique and inorganic carbon dose-response curves. Stable carbon isotope data of the diatoms are also presented. Observed rates of photosynthetic oxygen evolution were greater than could be accounted for by the theoretical rate of CO₂ supply from the uncatalyzed dehydration of HCO₃^? in the external medium, suggesting use of HCO₃^? as an inorganic carbon source. Data from the isotopic disequilibrium experiment demonstrate the use of both HCO₃^? and CO₂ for photosynthesis. Carbon isotope discrimination values support the use of HCO₃^? by the diatoms.     

The effect of positive assortative mating at one locus on a second linked locus

Summary A model for positive assortative mating based on genotype for one locus is employed to investigate the effect of this mating system on the genotypic structure of a second linked locus as well as on the joint genotypic structure of these two loci. It is shown that the second locus does not attain a precise positive assortative mating structure, but yet it shares a property that is characteristic of positive assortative mating, namely an increase in the frequency of homozygotes over that typically found in panmictic structures. Given any arbitrary genotypic structure for the parental population, the resulting offspring generation possesses a structure at the second locus that does not depend on the recombination frequency, while the joint structure of course does. In case assortative mating as well as linkage are not complete, there exists a unique joint equilibrium state for the two loci, which is characterized by complete stochastic independence between the two loci as well as by Hardy-Weinberg proportions at the second locus. For the second locus alone, Hardy-Weinberg equilibrium is realized if and only if gametic linkage equilibrium and an additionally specified condition are realized.     

An Isoenzyme Analysis of the Genus Phytomonas: Genetic, Taxonomic and Epidemiologic Significance

ABSTRACT We studied 31 Phytomonas stocks isolated from various hosts and a broad geographical range by isoenzyme electrophoresis (14 loci) and population genetics analysis. The total variability is considerable since many stocks share no allele. Population genetic analysis strongly suggests that Phytomonas zymodemes behave as natural clones, as already proposed by us for several other protozoan species. These clones should be considered as actual taxa in all applied studies. Latex plants and phloemic plants (coconut and palm trees) harbor distinct sets of clones; hence, latex plants studied in this article are probably not a reservoir for parasites of the coconut and palm tree.     

Rates of decay of linkage disequilibrium under two-locus models of selection

The effect of selection and linkage on the decay of linkage disequilibrium, D, is investigated for a hierarchy of two-locus models. The method of analysis rests upon a qualitative classification of the dynamic of D under selection relative to the neutral dynamic. To eliminate the confounding effects of gene frequency change, the behavior of D is first studied with gene frequencies fixed at their invariant values. Second, the results are extended to certain special situations where gene frequencies are changing simultaneously.A wide variety of selection regimes can cause an acceleration of the rate of decay of D relative to the neutral rate. Specifically, the asymptotic rate of decay is always faster than the neutral rate in the neighborhood of a stable equilibrium point, when viabilities are additive or only one locus is selected. This is not necessarily the case for models in which there is nonzero additive epistasis. With multiplicative viabilities, decay is always accelerated near a stable boundary equilibrium, but decay is only faster near the stable central equilibrium (with = 0) if linkage is sufficiently loose. In the symmetric viability model, decay may even be retarded near a stable boundary equilibrium. Decay is only accelerated near a stable corner equilibrium when the double homozygote is more fit than the double heterozygotes. Decay near a stable edge equilibrium may be retarded if there is loose linkage. With symmetric viabilities there is usually an acceleration of the decay process for gene frequencies near 1/2 when the central equilibrium (with = 0) is stable. This is always the case when the sign of the epistasis is negative or zero.Conversely, the decay ofD is retarded in the neighborhood of a stable equilibrium in the multiplicative and symmetric viability models if any of the conditions above are violated. Near an unstable equilibrium of any of the models considered,D may either increase or decay at a rate slower than, equal to, or faster than the neutral rate. These analytic results are supplemented by numerical studies of the symmetric viability model.     

The effect of gametic-phase disequilibrium on the prediction of response to recurrent selection in plants

Selection reduces additive genetic variation by generating gametic-phase disequilibrium, a phenomenon largely ignored when predicting response in plant breeding programs. The development of gametic-phase disequilibrium is here taken into account when predicting the response to selection for various schemes of recurrent selection applicable to plant populations. A general program permits prediction of response to selection from schemes of recurrent selection in which two or more rounds of selection occur in each cycle. An example from Sugar Beet, with alternate rounds of half-sib and S1 family selection, is illustrated. It is shown that failure to take into account the effects of gametic-phase disequilibrium can result in substantial overestimation of the response to selection as well as to changes in rank of the merits of alternative breeding schemes. For a given scheme, ignoring gametic-phase disequilibrium has only small effects on defining the optimum allocation of plots and the numbers of families tested.