THE BULLER PHENOMENON IN SCHIZOPHYLLUM COMMUNE: NUCLEAR SELECTION IN FULLY COMPATIBLE DIKARYOTIC-HOMOKARYOTIC MATINGS

Ellingboe , Albert H., and John R. Raper . (Harvard U., Cambridge, Mass.) The Buller Phenomenon in Schizophyllum commune: nuclear selection in fully compatible dikaryotic-homokaryotic matings. Amer. Jour. Bot. 49(5): 454–459. 1962.—Internuclear selection in fully compatible, dikaryotic-homokaryotic matings of isogenic strains is apparently controlled by the incompatibility factors and, in some cases, by extra-incompatibility factors. The relationship of the structure of the incompatibility factors to internuclear selection is not clear. Both types of nuclei from the dikaryon migrate into the homokaryon, and it appears that selection occurs in the final establishment of the new dikaryon.     

Abo blood groups and completed reproductive performance of rural Haryanavi couples: analysing measures of selection intensities

The possible differential effects of ABO blood group materno-paternal (fetal) incompatibility on completed reproductive performance were investigated on a sample of 100 couples (100 fathers and 100 mothers) from three villages in the Jind district of Haryana state, India. The average number of live births per mating couple was slightly higher for the incompatible matings (5.32) than the compatible ones (5.05). This advantage was offset by higher postnatal mortality in the former. Consequently, the average number of living children in the compatible matings (4.64) was higher than in the incompatible ones (4.18). With reference to individual ABO matings, the index of relative fertility (Irf) was the highest in A x AB followed by B x A type of incompatible matings. No decrease in live births in O x A and O x B incompatible matings was observed compared with their reciprocal compatible ones, i.e. A x O and B x O matings, as has been hypothesized in previous studies. The total pregnancy wastage was substantially higher in ABO-incompatible matings (24.59%) than compatible matings (8.45%). About 71% of the postnatal deaths took place within one year of the birth in the case of incompatible matings compared with 50% in the case of compatible matings. The study supports the hypothesis that selection is operative at the ABO locus as revealed by the measures of selection intensity. The loss of fitness in the present sample was associated with differential mortality. There were no differences in the proportions of average number of male live births in the compatible (0.55) and incompatible matings (0.58). However, in the individual mating types, there was some evidence of higher or lower proportions of male live births.     

Fungal fidelity: Nuclear divorce from a dikaryon by mating or monokaryon regeneration

Basidiomycete fungi perform fertilizations by incorporation of nuclei into a monokaryotic mycelium to establish a dikaryon. The dikaryon cannot incorporate another type of nucleus, but can still act as a nucleus donor in a dikaryon–monokaryon (di–mon) mating, known as the Buller phenomenon. Previously, it has been observed that: (1) in a particular di–mon mating, one of the nuclear types of the dikaryon generally performs better as a donor than the other, and (2) when nuclei from a dikaryon are separated to form monokaryons again (dedikaryotisation), recovery of monokaryons of the two nuclear types is usually unequal. In this study, we investigated if these two observations of asymmetry are functionally related. We tested this hypothesis by performing both di–mon matings and dedikaryotisation of dikaryons derived from five different monokaryons. When a single mechanism controls both processes, the nucleus better at fertilizing a monokaryon in a Buller pairing should also be recovered upon dedikaryotisation with a higher frequency. The results showed a hierarchical structure for recovery among nuclei in dedikaryotisation, but this hierarchy did not correspond to the fertilization success during di–mon mating. These findings thus show that the mechanism causing asymmetric regeneration of nuclei, is most likely not the same as the mechanism responsible for increased chance of fertilization in di–mon matings. We discuss the complexity of the interactions that occur during di–mon matings with regards to the mating type loci.     

ABO blood groups and fertility—with special reference to intrauterine selection due to materno-fetal incompatibility

The purpose of the present investigation was to study whether there is differential fertility between different mating types of ABO blood group system. Selective force which is operating through maternal-fetal incompatibility has been observed in the differential fertility between compatible and incompatible mating groups in the present sample of 183 families of Visakhapatnam town of Andhra Pradesh, India. The differences in the mean numbers of pregnancies as well as living children between the two major mating groups, compatible and incompatible are significant. The fertility rates of O fathers and O mothers were significantly higher than those in matings in which neither parents belongs to O. The selection is operating to reduce the gene ratio of A and to increase the gene ratios of O and B in this sample.     

THE KINETICS OF INITIAL NUCLEAR EXCHANGE IN COMPATIBLE AND NONCOMPATIBLE MATINGS OF SCHIZOPHYLLUM COMMUNE

The effect of mating-type factors on exchange of nuclei between mycelial fragments mated in liquid culture was studied. Evidence for exchange of nuclei was based on three criteria for detecting nuclei from both mating partners in mycelial fragments of the developing dikaryon or hetero-karyon. The kinetics of nuclear exchange were shown to be relatively independent of the method used. It was shown that the kinetics of nuclear exchange in the first 96 hr are different in the compatible (A41 B41 × A42 B42), common-A (A42 B42 × A42 B41), common-B (A41 B42 × A42 B42), and common-AB (A41 B41 × A41 B41) matings. In all four types of matings, the percentage of fragments possessing both types of nuclei 12-24 hr after mating is nearly equal. After this time, significant differences appear in the patterns for compatible vs. the three non-compatible matings and the common-A vs. the common-B and common-AB matings. The percentage of mycelial fragments possessing both types of nuclei throughout the 96-hr test period is similar for both the common-B and common-AB matings. The kinetics of nuclear exchange were shown to be independent of the particular mating-type alleles and nutritional markers used. When the efficiency of nuclear exchange in complete and minimal media was compared, it was shown that nuclear exchange occurred more rapidly and synchronously in minimal medium. This difference is not due to growth differences in the two media. These data indicate that the earliest mating interactions, i.e., hyphal anastomosis and nuclear exchange, are independent of the mating-type factors but that subsequent events are determined by these genes.     

Selection for Increased Mutation Rates with Fertility Differences between Matings

Previous studies of mutation modification have considered models in which selection is a result of viability differences that are sex symmetric. The results of a numerical study of a model in which selection is a result of fertility differences between mated pairs demonstrate that the type of selection to which a population is subject can have a significant impact on the evolution of various parameters of the genetic system. When the fertility of matings between individuals with different genotypes exceeds the fertility of at least some of the matings between individuals with the same genotype, selection may favor increased rates of mutation, in contrast to the results from all existing constant viability models with random mating and infinite population size. Increased mutation rates are most frequently favored when forward and back mutation occur at approximately equal rates and when the modifying locus is loosely linked to the selected locus. We present one example in which selection favors increased rates of mutation even though the selection scheme is reducible to one of differential viability between the sexes.     

Direct studies of dikaryotization in Schizophyllum commune

Compatible matings of Schizophyllum commune were performed on glucose-peptone-yeast extract medium appended with gelatin (18%) and studied by phase contrast microscopy during nuclear migration. Three categories of nuclear migration were observed. Type I involved a pulsatile jerking of the entire cytoplasmic contents of the hypha, changed direction periodically, and, during periods of cytoplasmic tranquility, the nucleus continued to migrate. Type II A migration of nuclei occurred in the absence of visible cytoplasmic flow. Both Type I and Type II A nuclear movements exceeded the hyphal growth rate by 10--20-fold. Type II B nuclear migration also occurred in the absence of visible cytoplasmic flow and the velocity was within the range of the hyphal growth rate. No specific organelles that were detected either directed or facilitated Type II A or Type II B nuclear movements. The nucleolus could either lead or trail relative to the direction of nuclear movement. Nuclear migration can be attributed to both cytoplasmic flow and self motility, depending upon the particular regions of the migration hypha in which it occurs.     

PHASE III OF WRIGHT'S SHIFTING BALANCE PROCESS AND THE VARIANCE AMONG DEMES IN MIGRATION RATE

Interdemic selection by the differential migration of individuals out from demes of high fitness and into demes of low fitness (Phase III) is one of the most controversial aspects of Wright's Shifting Balance Theory. I derive a relationship between Phase III migration and the interdemic selection differential, S, and show its potential effect on F_ST. The relationship reveals a diversifying effect of interdemic selection by Phase III migration on the genetic structure of a metapopulation. Using experimental metapopulations, I explored the effect of Phase III migration on F_ST by comparing the genetic variance among demes for two different patterns of migration: (1) island model migration and (2) Wright's Phase III migration. Although mean migration rates were the same, I found that the variance among demes in migration rate was significantly higher with Phase III than with island model migration. As a result, F_ST for the frequency of a neutral marker locus was higher with Phase III than it was with island model migration. By increasing F_ST, Phase III enhanced the genetic differentiation among demes for traits not subject to interdemic selection. This feature makes Wright's process different from individual selection which, by reducing effective population size, decreases the genetic variance within demes for all other traits. I discussed this finding in relation to the efficacy of Phase III and random migration for effecting peak shifts, and the contribution of genes with indirect effects to among‐deme variation.     

PEAK SHIFTS AND POLYMORPHISM DURING PHASE THREE OF WRIGHT'S SHIFTING-BALANCE PROCESS

The third phase of Wright's shifting-balance theory involves the export of adaptive gene combinations from one subpopulation to another. Previous results have demonstrated that this can occur at very low migration rates, but it has been argued that this simply reflects the ability of migration to overcome selection and fix any (even deleterious) alleles. Here, previous analyses are extended by concentrating on the critical balance between forward and reverse migration rates that still allows phase III to proceed. It is shown that selective advantage, dominance, recombination rate, and the number of loci all affect the ability of a genotype to invade and become fixed in a new subpopulation, but it is unlikely that phase III will occur in the absence of differential migration unless the invading genotype consists of a few dominant loci with a large selection advantage, spreading into a few populations of lower fitness. Therefore, as was envisioned by Wright, differential migration from more to less fit populations will be necessary for phase III to occur under most circumstances.     

THE DYNAMICS OF MITOCHONDRIAL MUTATIONS CAUSING MALE INFERTILITY IN SPATIALLY STRUCTURED POPULATIONS

Mitochondrial genomes are usually inherited maternally and therefore there is no direct selection against mutations that have deleterious effects in males only (mother’s curse). This is true in particular for mitochondrial mutations that reduce the fertility of their male carriers, as has been reported in a number of species. Using both analytical methods and computer simulations, we demonstrate that spatial population structure can induce strong selection against such male infertility mutations. This is because (1) infertile males may reduce the fecundity of the females they mate with and (2) population structure induces increased levels of inbreeding, so that the fitness of females carrying the mutation is more strongly reduced than the fitness of wild‐type females. Selection against mitochondrial male infertility mutations increases with decreasing deme size and migration rates, and in particular with female migration rates. On the other hand, the migration model (e.g., island or stepping stone model) has generally only minor effects on the fate of the mitochondrial mutations.     

Migration rate of rabbit bone-marrow stromal cells and rabbit dermal fibroblasts in different gels and activity of their MMPS

This work deals with the study of the migration rate of rabbit cells in collagen and fibrin gels. It has been shown that dermal fibroblasts more actively migrate in collagen gel, whereas in fibrin gel bone-marrow stromal cells do. In the study of activities of matrix metalloproteinases (MMPs) that are synthesized by cells in the process of cultivation, MMP-2 activity in cells of both types was established as being higher during migration in collagen gel, while MMP-9 activity was so during migration in fibrin gel. The different cell migration rates may be due to the cell properties, to the activity of their synthesized MMP, and to the effect up this process of the cell microenvironment (collagen or fibrin).     

Experimental evolution in Heterandria formosa, a livebearing fish: group selection on population size

Group selection has historically been an important and controversial subject in evolutionary biology. There is now a compelling body of evidence, both theoretical and experimental, that group selection not only can be effective, but can be effective in situations when individual selection is not. However, experiments in which true population-level traits have been shown to evolve in response to group selection are currently limited to two species of flour beetle in the genus Tribolium and RNA viruses. Here we report the results of an experiment wherein we imposed group selection via differential extinction for increased and decreased population size at 6-week intervals, a true population-level trait, in the poeciliid fish Heterandria formosa. In contrast to most other group selection experiments, we observed no evolutionary response after six rounds of group selection in either the up- or down-selected lines. Populational heritability for population size was low, if not actually negative. Our results suggest that group selection via differential extinction may be effective only if population sizes are very small and/or migration rates are low.     

Consanguinity in South America: demographic aspects

A sample of 53,552 nonmalformed liveborn infants was ascertained by the Latin-American Collaborative Study of Congenital Malformations between 1967 and 1996. The mean consanguinity rate was 0.96%, with significantly higher values in Brazil and Venezuela, and lower in Argentina. Low paternal education and occupation levels were positively associated with consanguinity. First-cousin matings represented almost half of all consanguineous couples. The consanguinity was mainly of more closely related types in Brazil, while in Venezuela more remote types predominated. This could reflect differences in migration patterns and rates between these two countries.     

Differential response to abiotic conditions and predation risk rather than competition avoidance determine breeding site selection by anurans

Co‐existence of species has been a central debate in ecology for decades but the mechanisms that allow co‐existence are still heatedly disputed. The main paradigms have shifted among the importance of competition, predation and abiotic conditions as determinants of community structure. Differential habitat selection is considered to reduce competition and hence allow co‐existence. Our goal was to test hypotheses regarding how breeding site use of a population that was patchily distributed on a dynamic floodplain may facilitate coexistence: 1) do species co‐occur randomly or do they occur more or less often than expected by chance? 2) Do species use the same habitat types in equal proportions or do they use them differentially? 3) If they use habitat types differentially, is this differential use related to abiotic and biotic conditions? 4) Does interspecific competition predict breeding site use or do abiotic conditions and predation risk better predict habitat use? We collected presence/absence (i.e. detection/nondetection) data of egg clutches and larvae of four pond‐breeding anuran species during a two year study at a total of 353 ponds. We used site occupancy models and model averaging techniques to predict breeding site selection in relation to habitat types, abiotic and biotic factors. These parameters were corrected for imperfect detection of species. The rates of co‐occurrence were consistently higher than expected by chance. Species differed in the use of the main habitat types. Habitat types that were used by multiple species were used in a species‐specific manner in relation to both abiotic conditions and predation risk. Species preferred ponds where other species and fish were present. Although niche differentiation in breeding site selection is evident, our results do not support the pervasive role of competition avoidance in governing current breeding site selection. We conclude that differential habitat use and differences in response to abiotic conditions and predation risk can override competitive interactions, thereby facilitating local co‐existence and high species diversity.     

DISPERSAL AND PLANT MATING SYSTEMS: THE EVOLUTION OF SELF-FERTILIZATION IN SUBDIVIDED POPULATIONS

Intermediate rates of self-fertilization can be evolutionarily stable when the progeny of self-fertilization events are less successful migrants than those of outcrossing events, unless self-fertilization reduces an individual's contribution to the pollen pool by an amount equal to the rate at which it self-fertilizes. This result holds regardless of whether pollen or diaspores are more widely dispersed. The differential migration of selfed and outcrossed progeny may be a result of differential establishment with comparable rates of dispersal, or it may be a result of differential dispersal rates. In the first case, detailed predictions concerning the evolutionarily stable selfing rate can be made. In the second case, only qualitative predictions are possible in the absence of specific assumptions about how the migration rate is affected by the average selfing rate in each subpopulation.     

Why mitochondrial genes are most often found in nuclei

A very small fraction of the proteins required for the propagation and function of mitochondria are coded by their genomes, while nuclear genes code the vast majority. We studied the migration of genes between the two genomes when transfer mechanisms mediate this exchange. We could calculate the influence of differential mutation rates, as well as that of biased transfer rates, on the partitioning of genes between the two genomes. We observe no significant difference in partitioning for haploid and diploid cell populations, but the effective size of cell populations is important. For infinitely large effective populations, higher mutation rates in mitochondria than in nuclear genomes are required to drive mitochondrial genes to the nuclear genome. In the more realistic case of finite populations, gene transfer favoring the nucleus and/or higher mutation rates in the mitochondrion will drive mitochondrial genes to the nucleus. We summarize experimental data that identify a gene transfer process mediated by vacuoles that favors the accumulation of mitochondrial genes in the nuclei of modern cells. Finally, we compare the behavior of mitochondrial genes for which transfer to the nucleus is neutral or influenced by purifying selection.     

Kinesin-1 and dynein at the nuclear envelope mediate the bidirectional migrations of nuclei

Kinesin-1 and dynein are recruited to the nuclear envelope by the Caenorhabditis elegans klarsicht/ANC-1/Syne homology (KASH) protein UNC-83 to move nuclei. The mechanisms of how these motors are coordinated to mediate nuclear migration are unknown. Time-lapse differential interference contrast and fluorescence imaging of embryonic hypodermal nuclear migration events were used to characterize the kinetics of nuclear migration and determine microtubule dynamics and polarity. Wild-type nuclei display bidirectional movements during migration and are also able to roll past cytoplasmic granules. unc-83, unc-84, and kinesin-1 mutants have severe nuclear migration defects. Without dynein, nuclear migration initiates normally but lacks bidirectional movement and shows defects in nuclear rolling, implicating dynein in resolution of cytoplasmic roadblocks. Microtubules are highly dynamic during nuclear migration. EB1::green fluorescence protein imaging demonstrates that microtubules are polarized in the direction of nuclear migration. This organization of microtubules fits with our model that kinesin-1 moves nuclei forward and dynein functions to move nuclei backward for short stretches to bypass cellular roadblocks.     

EQUILIBRIUM ANALYSIS OF SEXUAL SELECTION IN DROSOPHILA MELANOGASTER

Several models for sexual selection, both by male-male competition and female choice, predict that a character which covaries with mating success should be near an equilibrium where the intensity of sexual selection opposes viability selection. This prediction was used to design experiments for estimating the intensity of sexual and viability selection on wing length in a recently captured population of Drosophila melanogaster. Observations of matings by males color-marked for wing length indicated that the standardized sexual selection differential on wing length was 0.24 under a wide range of effective sex ratios. After estimating the heritability of wing length to be 0.62, the expected standardized response due to sexual selection was calculated as 0.15 (SE = 0.15). The response due to viability selection was then estimated by comparing wing lengths of progeny of flies that had been randomly mated, thereby preventing sexual selection, with progeny of flies that had been allowed to acquire mates in a mass-mating chamber. The results support an equilibrium model in that the standardized response due to viability selection (?0.31, SE = 0.08) was opposite in sign and similar in magnitude to the estimated response due to sexual selection. Observations of females orienting in front of males which differed in wing length indicated that the mating advantage accruing to long-winged males was not due to female choice. Instead, male-male competition in which the larger of two randomly chosen males succeeded in mating, explains the observed sexual selection. An experimental analysis of genotype-environment interaction revealed that larval density had a nonlinear effect on mean wing length within sibships. If a population is displaced from equilibrium, therefore, the evolutionary trajectory of mean wing length will depend both on the intensity of selection and the environment in which that selection is operating.     

POLYMORPHISM IN RINGED PLOVERS

The rather similar ringed plovers Charadrius hiaticula and C. semipalmatus represent essentially allopatric, circumpolar populations which overlap in the breeding season only in a narrow zone in the eastern Canadian Arctic. In eight fiords on eastern Baffin Island, semipalmatus was the more numerous by a factor of almost three. Both plovers occurred together, although not in equal numbers, in the same wide range of habitats. Pair formation in both plovers apparently took place at the same time, but for more than half of the individuals, not in the same geographic area. The majority of the hiaticula individuals arrived after most semipalmatut, and the late arriving hiaticula appeared to be paired upon reaching Baffin Island. There was an overlap in their arrival times of about five days. Mixed hiaticula-semipalmatus courtship groups occurred during this period. Pair forming activities of hiaticula and semipalmatus appeared similar but were not studied closely. Analysis of the non-overlapping territories of two populations suggested that the plovers did discriminate one type from the other for there was a tendency for unlike pairs to nest side by side. The significance of this odd pattern is not known. Analysis of the mensural and plumage characteristics of adult plovers from the overlap zone seemed to indicate that gene flow between the two was absent and that they were behaving as distinct species. However, mixed pairs occurred in five out of eight fiords. Considering the populations of all fiords, the number of mixed pairs found (20) was significantly greater than theoretical expectation, which allowed for an occasional mixed mating since a choice of mate was not always possible. All mixed matings produced mixed broods. The young were either hiaticula or semipalmatus. No intermediates occurred. Moreover, 10 pairs in which both birds were pheno-typically “semipalmatus” produced both semipalmatus and hiaticula-type chicks. No intermediates were detected. Egg and chick mortality probably prevented the detection of more such cases. All hiaticula x hiaticula matings produced only hiaticula-type chicks. The inheritance data indicated that hiaticula and semipalmatus are morphs of a single polymorphic species. If a switch gene is operating, it may have originally controlled only toe-webbing, and the other differences may have accumulated as the result of geographic isolation. That differences in plumage pattern and body size are greatest where separation was probably longest supports this hypothesis. One possible model considered semipalmatus-type individuals to be either SS or Ss and hiaticula-types to be ss. Accordingly, all semipalmatus-types involved in mixed matings with hiaticula- types would have been Ss individuals because all such matings produced chicks of both types. Circumstantial evidence suggested that during the last 80 years, the relative number of the two types in the overlap area has shifted in favour of the semipalmatus- type. This might have been the result of selective factors intrinsic to the polymorphism.