Co-adapted Gene Research Articles

Genetic considerations in transfers and introductions of scallops

As the potential of scallop aquaculture becomes increasinglyrealised, transfers (movement within a species' range) andintroductions (movement outside a species' range) of scallops arebecoming more common. To predict the genetic consequences oftransfers, information on genetic differences between source andrecipient populations is vital. Morphological, allozyme and DNAbased data on genetic differentiation of scallop populations andscallop sub-species are presented and discussed. Otherconsiderations are the numbers of individuals transferred andwhether they are wild stock or hatchery product. Loss of geneticdiversity is difficult to avoid in hatchery conditions althoughthere are ecological advantages to using disease-free hatcheryseed. Mitochondrial DNA data indicating significant geneticconsequences of the introduction of Argopecten irradians fromthe USA to China are discussed and compared with data onPatinopecten yessoensis introduced from Japan to Canada.Potential risks and consequences of hybridisation should beexperimentally assessed before introductions of scallops arecarried out. Hybridisation is unpredictable and can lead to lossof genetic diversity or breakdown of co-adapted gene complexes.The use of sterile triploid scallops for introductions to avoid hybridisation and reduce ecological impact has merit butreversion to diploidy may occur.

Genetic Structure of Coral Reef Organisms: Ghosts of Dispersal Past

SYNOPSIS. Molecular genetic studies are revealing the presence of cryptic taxa, and patterns of gene flow in coral reef species, that do not correspond to present day ocean circulation patterns. Concordant borders of genetic inhomogeneity in several taxa emphasise the influence of historical barriers to gene flow. The persistence of genetic differences between sites apparently connected by present-day currents provides evidence for lack of effective contemporary gene exchange. A review of the limited data available to date cannot be conclusive, but suggests that present patterns of genetic variation in the Indo-Pacific have resulted from highly pulsed dispersal events associated with range expansion during interglacial periods. Thus, population genetic structure appears to be dominated by events associated with global climate change and sea level fluctuation during the last 1–3 million years, rather than vicariant geological events in the early Caenozoic. Regional speciation outside the tropical Indo-West Pacific and movement of these species into that region may have played a more important role in producing diversity in that region than traditionally recognised. Some genetic variants have arisen before, and have persisted through, several cycles of climate change. The genetic structure of populations is likely to have been maintained for several thousand years after they were first established, during or immediately after range expansion, by the occurrence of co-adapted gene complexes of some form, and because of more limited opportunity for dispersal than has been assumed to date.

Sampling strategy for a core collection of Peruvian quinoa germplasm

Quinoa (Chenopodium quinoa) is an Andean crop with a high potential for cultivation under temperate agricultural conditions. A new quinoa cultivar for such an environment requires plant characteristics that may be available primarily largely in genetic resources held in gene banks. A core collection may simplify management and enhance the utilisation of quinoa genetic resources. This paper describes the development of a core subset of the whole quinoa gene bank (1029 accessions) of the Universidad Nacional del Altiplano (UNAP). All accessions available in this gene bank have location and altitude descriptors and a partial description for qualitative and quantitative descriptors. The core collection (103 accessions) contains chosen ecotypes or landraces that capture most of the genetic variability available in this Peruvian germplasm. The accessions were selected for the core collection based on a geographically stratified non-overlapping sampling procedure. The number of accessions that were allocated to the core subset was determined using a proportional method adjusted by the relative importance of the quinoa crop in each geographical cluster as determined by its acreage. The sampling method also considered the morphological diversity within four geographical clusters of at least 100 accessions. The multivariate pattern of morphological variation was defined within each of these clusters by independent principal component analyses. A comparison of phenotypic diversity between the entire collection and its core subset confirmed that the proper sampling strategy for this core collection of Peruvian quinoa germplasm had been applied. The most important phenotypic correlations between quantitative descriptors observed in the entire collection, which may be under the control of co-adapted gene complexes, were also preserved by the core collection. The most comprenhensive quinoa core collection should consider accessions from other gene banks in Bolivia and Ecuador, a few accessions from coastal Chile and wild sympatric cross-compatible Chenopodium species. This core collection will be a point of entry to the proper exploitation of the genetic resources available in respective quinoa gene banks.

Evolution of competitive fitness in experimental populations of E. coli: what makes one genotype a better competitor than another?

An important problem in microbial ecology is to identify those phenotypic attributes that are responsible for competitive fitness in a particular environment. Thousands of papers have been published on the physiology, biochemistry, and molecular genetics of Escherichia coli and other bacterial models. Nonetheless, little is known about what makes one genotype a better competitor than another even in such well studied systems. Here, we review experiments to identify the phenotypic bases of improved competitive fitness in twelve E. coli populations that evolved for thousands of generations in a defined environment, in which glucose was the limiting substrate. After 10,000 generations, the average fitness of the derived genotypes had increased by approximately 50% relative to the ancestor, based on competition experiments using marked strains in the same environment. The growth kinetics of the ancestral and derived genotypes showed that the latter have a shorter lag phase upon transfer into fresh medium and a higher maximum growth rate. Competition experiments were also performed in environments where other substrates were substituted for glucose. The derived genotypes are generally more fit in competition for those substrates that use the same mechanism of transport as glucose, which suggests that enhanced transport was an important target of natural selection in the evolutionary environment. All of the derived genotypes produce much larger cells than does the ancestor, even when both types are forced to grow at the same rate. Some but not all, of the derived genotypes also have greatly elevated mutation rates. Efforts are now underway to identify the genetic changes that underlie those phenotypic changes, especially substrate specificity and elevated mutation rate for which there are good candidate loci. Identification and subsequent manipulation of these genes may provide new insights into the reproducibility of adaptive evolution, the importance of co-adapted gene complexes, and the extent to which distinct phenotypes (e.g., substrate specificity and cell size) are affected by the same mutations.

Developmental instability in gynodioecious Teucrium lusitanicum

Developmental instability was assessed in two geographical races of Teucrium lusitanicum using morphometric measures of vegetative and reproductive structures. T. lusitanicum is a gynodioecious species. Male sterile (female) individuals showed greater developmental instability at all sites. Plants located inland had higher developmental instability of vegetative characters and lower developmental instability of reproductive characters than coastal plants. These results support the contentions that (1) developmental instability is affected more by the disruption of co-adapted gene complexes than by lower heterozygosity, and (2) different habitat characteristics result in the differential response of vegetative and reproductive structures.

SELECTION FOR NATIVE CHARACTERS IN HYBRIDS BETWEEN TWO LOCALLY ADAPTED PLANT SUBSPECIES.

Gene exchange between locally adapted plant populations can have significant evolutionary consequences, including changes in genetic diversity, introduction of adaptive or maladaptive traits, disruptive of coadaptive gene complexes, and the creation of new ecotypes or even species. The potential for introgression between divergent populations will depend on the strength of selection against nonnative characters. Morphologically variable F2 hybrids of two Gilia capitata subspecies were used to evaluate the strength of phenotypic selection and the response to selection in the home habitats of each subspecies. At both sites, traits diagnostic of the subspecies were subject to significant phenotypic selection, probably mediated by direct selection on unmeasured correlated characters. Phenotypic selection favored native morphologies in all but a single case; leaf shape of one subspecies was favored in both habitats. The strength of selection varied between sites, with one site selecting more strongly against nonnative characters. Offspring of the F2 hybrids showed a significant evolutionary response to selection when grown in a common environment. Evolution was in the direction of similarity with the subspecies native to the site where selection was imposed. This result reveals that native character states are adaptive and suggests that selection will maintain native morphologies even after a substantial influx of genes from an ecologically and morphologically distinct, and locally adapted subspecies.

Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans.

We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.

Genetic consequences ofin situandex situconservation of forest trees

To counteract loss of genetic diversity crucial for current and future tree improvement, tree breeders have conserved forest genetic resources in situ in their natural ecosystems in protected areas or ex situ in plantations, seed orchards, and breeding arboreta. This article reviews the genetic consequences of these two conservation methods in terms of single-locus and multilocus population structure from electrophoretic studies of natural forests and breeding populations. Although natural forest populations have maintained high level of genetic diversity and exhibited low level of population differentiation, loss of genetic diversity would occur during the entire conservation process, from population establishment to management of breeding and production populations. Since forest trees are still at their earliest stage of domestication in Canada, loss of genetic diversity comes primarily from the initial sampling process during population establishment. We discuss the optimal sampling strategy during population establishment to conserve common and widespread alleles, common and localized alleles, rare and widespread alleles, and rare and localized alleles. We also discuss three methods for studying the multilocus structure of forest trees and show how such information would be useful for conserving co-adapted gene complexes. We conclude that being small and maintained in controlled environments, ex situ conserved populations would retain less genetic diversity than in situ conserved forest populations. While ex situ conservation is operationally convenient for the short-term gains in tree improvement, we believe in situ conservation is essential for renewing the genetic diversity to meet the changing environments of an uncertain future.

Heterosis in F2-Generations of Drosophila-Melanogaster

Three crosses among inbred lines of Drosophila melanogaster generated by eight generation full-sib mating, three Australasian population crosses, and two Australian/English population crosses were raised to the F2 generation, to examine evidence of hybrid breakdown. Inbred line crosses produced F2 generations that retained, on average, 54% of the F1 heterosis, which was not significantly different from the retention of 50% F1 heterosis expected under a dominance model of heterosis. Australasian population crosses produced F2 generations with significantly less than 50% of the F1 heterosis, while crosses between Australian and English populations produced a F2 generation that generally performed worse than midparent purebred populations. These results imply epistatic gene action. Because inbred lines contain random combinations of genes, recombination does not necessarily produce hybrid breakdown. However, in crosses between natural populations, recombination is likely to disrupt co-adapted gene combinations leading to poor F2 performance. Populations which rarely exchange migrants are more likely to have alternative gene combinations and to show hybrid breakdown than populations which regularly exchange migrants. The implications of these findings for livestock crossbreeding programs is discussed.

Absence of Y-chromosome introgression across the hybrid zone between Mus musculus domesticus and Mus musculus musculus.

SummaryA cloned Y-specific sequence (Bishop et al. 1985) was used as a diagnostic probe to distinguish between Mus musculus domesticus and Mus musculus musculusY-chromosomes. Analysis of the RFLPs obtained with genomic DNA isolated from wild mice caught along the contact zone between M. m. domesticus and M. m. musculus in Bulgaria and Denmark showed that the Y-chromosome flow between the two semi-species is very limited. The degree of Y-chromosome penetration was compared with that of seven diagnostic autosomal loci and the mitochondrial DNA. Breeding experiments showed that the lack of Y-chromosome introgression from one semispecies to the other was not due to a major hybrid breakdown. The results suggest that the disruption of differentiated co-adapted gene systems could play a role in limiting Y-introgression.

Area effect in south western European green frogs (Amphibia, Ranidae)

Abstract Enzyme polymorphism in SW European green frogs (Rana esculenta Complex) show concordant patterns of very high differentiation between populations. Demes sampled in N Italy are all identifiable by at least one diagnostic locus, while those from Spain and S. France (Rana perezi), differ from the latter over five such loci. No single individual was found to be heterozygous for all non‐mono‐morphic loci. This pattern of variation is correlated to interdemic morphological differentiation and denotes the accumulation of major genetic differences resulting in local coadapted gene complexes. It is therefore interpretable in terms of area effect and stasipatric model of evolution. The genetic structure of these SW European populations of green frogs is not compatible with the hybridogenetic model.

Evolution of an instinct under long-term divergent selection for geotaxis in domesticated populations of Drosophila melanogaster..

Long-term divergent selection for geotaxis in lines of Drosophila melanogaster (the fruit fly) is described. After 26 years (600+ generations) of intermittent selection, the mean geotactic scores now remain stable upon relaxed selection, a result suggesting that evolutionary changes have occurred in these lines. Because the stability is not due to genetic fixation (homozygosity) of the lines, it must be due to either close linkage of genes associated with geotaxis (which would not result in enduring change) or the development of new coadapted gene complexes utilizing genes associated with extreme geotaxis expression (which should result in enduring change). Better evidence for the latter hypothesis is obtained from the low, than from the high, line. The notions of coadaptation and genetic homeostasis are considered, as well as the prospective use of the geotaxis lines to study such concepts experimentally. The geotactic behavior expressed in each line is defined as "instinctive": It increases reproductive success and is characteristic of a population.

Coadaptive Gene Complexes in Incipient Species of Hawaiian Drosophila

The degree of postmating isolation existing among allopatric populations belonging to two species of endemic Hawaiian Drosophila was investigated. Drosophila grimshawi is unusual among picture-winged Hawaiian Drosophila in that it is widely distributed in the archipelago with populations on the islands of Kauai, Oahu, Lanai, Molokai, and Maui. The second species, D. pullipes, is very closely related and is confined to the island of Hawaii. Kauai and Oahu D. grimshawi and D. pullipes are known to be ovipositional specialists, utilizing only one genus of plant. Populations of D. grimshawi from Lanai, Molokai, and Maui are generalists and oviposit on several plants of a number of different families. Hybridizations among stocks representing four of the six allopatric populations (i.e., Oahu, Maui, and Molokai D. grimshawi and D. pullipes) were completed in all reciprocal combinations; and, in addition, Kauai D. grimshawi was crossed to Oahu D. grimshawi. The F1 hybrid progeny were used to produce an F2 and backcrosses to parental males and females. Although crosses between ecologically different D. grimshawi stocks resulted in full F1 male fertility, breakdown was observed in the F2 and backcross males. This was not observed in crosses between ecologically similar populations. Fertility breakdown in these intraspecific crosses was consistently greatest in the F2's, followed by the backcross to the parental males, and least in the backcross to the parental females. This progressive breakdown, based on the assumption of no crossover recombination in Hawaiian Drosophila males, was attributed to the breakup of two differing sets of coadaptive gene complexes. It is further suggested that the coadaptive gene complexes of populations with the same adaptive peaks are identical or at least similar enough so as not to have yet evolved reproductive isolation. The D. pullipes stock was found to produce completely sterile F1 males when hybridized with all stocks except Molokai, in which approximately 70% of the F1 males were fertile. Added to the hybrid sterility, crosses involving D. pullipes females showed a reduction in hybrid viability.

The inheritance of pheromone production in the sulphur butterflies Colias eurytheme and C. Philodice

The inheritance of male pheromone production in the sulphur butterflies Colias eurytheme and C. philodice was determined by gas chromatographic analyses of wing extracts from various genotypes derived from interspecific crosses. A wing ultraviolet-reflectance pattern (found only in C eurytheme males) which is controlled by an X-linked gene served as a marker for determining the relative influence of this chromosome v. the autosomes on pheromone production. The data indicate that production of the most important C. philodice wing compounds, three different n-hexyl esters, is controlled by one or more autosomal genes that are at least codominant. The X-chromosome carries most of the genes or the gene controlling production of the most important C. eurytheme compound, 13-methyl heptacosane. Expression of this compound in hybrids displays a codominant pattern. Because the wing ultraviolet-reflectance pattern of male C. eurytheme is an important courtship signal, it appears that all of the genes controlling the male courtship signals of this species are inherited as a co-adapted gene complex on the X-chromosome. The evolution of X-chromosomal control of both the visual (wing UV-reflectance pattern) and olfactory (13-methyl heptacosane) components of the courtship communication system of C. eurytheme is discussed and compared to the mode of inheritance of other communication systems.

Chain Processes in Chromosomal Speciation

White,M.J. D. (Department of Population Biology, Research School of Biological Sciences, Australian National University, P.O. Box 475 Canberra City, ACT, 2601. Australia) 1978. Chain processes in chromosomal speciation. Syst. Zool. 27:285–298.—In many groups of animals of restricted vagility very closely related species not only differ in karyotype, but exhibit very extensive differences, due to the establishment of “chains” of several or many structural chromosomal changes. These cases, which until recently appeared anomalous or inexplicable, can now be used to interpret a basic feature of the “stasipatric” mode of spe-ciation (parapatric speciation of some authors). It is suggested that an important reason for the establishment of these chromosomal rearrangements is their role in protecting coadapted gene complexes (“area effects”) from disruption by introgression from neighboring populations. A model, based on the mouse (Mus musculus) populations of Italy and Switzerland studied by E. Capanna, A. Gropp, and their collaborators, is proposed; it relies on sequential establishment of chromosomal rearrangements, each within the range of the previous one. By this means the genetic isolation of the area population is progressively reinforced.

Interest of mice in conspecific male odours is influenced by degree of kinship.

SMELL affects many aspects of the social and sexual behaviour of rodents1,2. For example, fighting in pairs of male mice is increased if one of the participants is made to smell unfamiliar by rubbing it with the urine of a strange male3. It has also been suggested that olfactory cues play an important part in mate choice4. In both rats5 and mice6, females which have been reared by artificially scented mothers have, when adult, been found to prefer males treated with this same scent to normal individuals. By contrast, Mainardi7 has found that female mice, reared in the presence of their fathers, later preferred to mate with males of a different strain. The difference between these two results may stem from the degree of strangeness involved, females preferring males which smell slightly unfamiliar rather than very familiar or totally strange. The results of the experiments using artificial odours suggest that early learning provides the yardstick by which unfamiliarity is judged5,6. Such a system may have an important role in assortative mating, enabling females given a choice to mate with those males by whom they are likely to conceive the best-adapted offspring. On this hypothesis females might be expected to prefer a small degree of unfamiliarity as an adaptation which minimises the chance of inbreeding. They might also be expected to reject males which smell very strange. At an extreme, this would avoid interspecific hybridisation, but it may also be disadvantageous within a species for very dissimilar individuals to mate as this would be likely to lead to the break-up of co-adapted gene complexes and thus the production of offspring of lower fitness8,9. We report here our examination of these predictions. Our data suggest that the olfactory preferences of female mice are consistent with this hypothesis.

The lack of evidence for co-adaptation in crosses between geographical races ofDrosophila subobscuraColl.

1. The paper described an attempt to see whether differences in co-adaptation between populations ofDrosophila subobscuraare related to the distance between them. The mean and the variance of body-size, development time and survival were recorded on parent populations and the F1and F2of various crosses to test for heterosis in the F1and decline in performance or greater variance in the F2, which might indicate the break-up of co-adapted gene arrays. Comparisons were carried out at different temperatures and on a variety of larval diets, especially sub-optimal ones in which the larvae were grown on synthetic media. A large number of wild flies were caught at sites separated by about 10 miles along a transect of southern Scotland; these comprised one series of comparisons. For more distant crosses flies were caught at sites in southern England, Denmark, Switzerland and Israel.2. There were well-defined differences in body-size, and, to a lesser degree, development time between populations from more widely separated localities and these showed evidence of a cline, northern populations having larger body-size. The difference in size between the Scottish and Isreal populations is about 20%.3. There was no evidence of differences in co-adaptation between populations even in crosses between populations from sites as far apart as Scotland and Israel. The F1's were always close to the mid-parent values and there was no evidence of breakdown in the F2nor of increased variability.4. There was hardly any evidence of gene-environment interaction either with respect to different diets or to different temperatures.5. Records of body-size on flies caught in the wild showed that they are extremely variable, indicating great variation in larval nutrition. Under natural condition stability of growth in body-size is conspicuously lacking in this species.6. An additional test of co-adaptation was based on the between-family variance of abdominal bristle number of intra- and inter-population matings in the two most widely separated populations. There was no evidence of greater variance in the inter-population series.7. To test for possible differences in breeding structure, the response to inbreeding was determined for two widely separated populations ofD. subobscuraand a long-established cage population ofD. melanogaster, on an unrestricted larval diet and also on several different kinds of sub-optimal diets. There was little or no sign of consistent differences between the species in their response to inbreeding.8. This test revealed differences between the two species in their minimum requirements for particular nutrients.subobscurais less able thanmelanogasterto withstand lower levels of protein and survival is particularly reduced. On the other hand,melanogasterhas a considerably higher requirement for choline. Where there are apparent differences between the species in the average effect of inbreeding, the inbreeding effect is greater on the relatively more sub-optimal diet.9. Comparison of the performance of the immediate descendants of wild flies with those derived from the same site, but kept in the laboratory for some twenty generations, failed to show any differences on several different diets and so there was no evidence that adaptation to laboratory conditions was important.10. The lack of evidence for co-adaptation apparently conflicts with what has been claimed for other species. Such differences are discussed.

Crossing Over and Inversions in Coadapted Systems

Crossing over between the ST gene arrangement of D. pseudoobscura and various third chromosome inversions has been analyzed and considered in light of the triad hypothesis of Wallace (1953). It has been shown that an inversion's role in a coadapted system cannot be predicted from cytological evidence alone without confirmatory crossover data. This was demonstrated for the Standard/Pikes Peak heterozygote by the presence of only one double crossover in the total of 6105 flies. The relationship of this finding to the formation of stabilized coadapted gene combinations between the ST and PP arrangements is discussed. Since Pikes Peak is a large simple inversion with respect to Standard, two strand double crossovers are expected. This almost complete lack of crossing over cannot be explained on the basis of any simple mechanism of crossover suppression. In addition, data are presented for the crossing over between ST and a newly discovered simple, third chromosome inversion.