Large Breeding Populations Research Articles

The impact of the commercial lobster fishery on seabirds at the tristan da Cunha Islands, South Atlantic Ocean

Observations were made on the effect of the commercial rock-lobster Jasus tristani fishery on seabird populations at the Tristan da Cunha Islands, central South Atlantic Ocean. The most serious impact of the fishery is the large number of seabirds killed when dazzled by the ships' lights at night. These night-strikes affect thousands of individuals of eight or more species annually. Recommendations are made to reduce this largely avoidable mortality. The killing of rockhopper penguins Eudyptes chrysocome for baiting lobster traps has ceased. Indirect effects on seabird prey populations resulting from rock-lobster exploitation are likely to be negligible, because there are no inshore-feeding birds that feed extensively on benthic organisms. Dietary supplementation also is unimportant because few seabirds scavenge from the fishery relative to the large breeding populations occurring at the islands. Dumping of refuse and broken gear should be discontinued; it entangles marine birds and mammals, and is aesthetically unacceptable. Continuous vigilance is essential to keep the ships rodent-free, because their accidental introduction to the uninhabited islands poses the greatest threat to the island faunas.

Population fluctuations and survival of Great Tits Par us major dependent on food supplied by man in winter

The 16‐year data of a Great Tit population, breeding and overwintering in a nest box area at Oulu, northern Finland, were studied to reveal the factors governing the annual fluctuations in density and local survival. The breeding density at Taskila was high after a cold spring (March‐April) and a warm August the year before was also followed by a large breeding population. A similar relationship resulted for the local survival of fledglings, and there was a tendency for immigrant yearlings to settle to breed in large numbers when the spring was cold. In the north, Great Tits cannot survive the winter in the forests but gather around feeding tables near human settlements. A cold spring evidently suppresses the birds' dispersal before the breeding season. For a population which is time‐limited, it is adaptive to remain in order to breed nearby instead of searching for territories out in the forests when conditions become favourable late in the spring. Thus, the negative correlation between spring temperature and the size of the following breeding population may be the result of a spatial distribution of juveniles achieved by behavioural responses to unfavourable conditions. The causality behind the high survival of resident juveniles after a warm August may be attributable to food availability during the time when the birds undergo their postjuvenile moult, which is an energy‐demanding process. The survival rate of breeding adults was highly dependent on the intensity of nightly predation upon individuals roosting in nest boxes. Stoats Mustela erminea, the prime predators, preyed on tits when the populations of their main prey species, small rodents, were low. When the effect of predation was eliminated, the density‐dependent survival of adult birds disappeared. During low predation years the adult survival rate was negatively related to the number of fledglings produced per pair; this suggests that investment in descendants is a substantial survival cost for reproducing individuals.

Dry matter digestibility in vitro of leaf and stem of buffel grass (Cenchrus ciliaris) and related species and its relation to plant morphology and anatomy

Twenty-one genotypes of buffel grass (Cenchrus ciliaris), and six other genotypes, viz. C. setigerus, C. Pennisetiforrnis and C. ciliaris x C. setigerus hybrids, were examined for dry matter digestibility in vitro (DMD) of recently expanded leaf and stem at anthesis. Differences in DMD were related to morphological and anatomical characteristics of the tissues.Leaf DMD ranged from 64.0 to 73.4%0 and stem DMD (internodes 2-4 from the top) from 47.4 to 61.7%; genotypes of high leaf DMD generally had high stem DMD (r= 0.68). Leaf DMD was negatively correlated with the proportion of thick-walled tissues in leaf cross-section (r= -0.76), leaf dry weight (r= 0.77 ) and specific leaf weight (r= -0.82). Stem DMD was inversely related to the proportion of lignified tissue in stem cross-section (r= -0.55), height of flowering stem (r= -0.51) and total number of vascular bundles in the stem section (r= -0.55). Genotypes with a high DMD tended to have lax leaves and small vegetative tillers.The main commercial cultivars of buffel, viz. American, Biloela and Gayndah, were 4-6% units (leaf) and 2.5-7% units (stem) lower in DMD than the best genotype, indicating potential for improvement of digestibility. Specific leaf weight and numbers of vascular bundles in stem cross-section, which arc easily and quickly measured and have a heritability comparable to that for DMD, could be useful criteria for preliminary assessment of a large breeding population.

Juvenile survival and recruitment in a population of Weddell seals (Leptonychotes weddellii) in McMurdo Sound, Antarctica

Survival and recruitment of Weddell seal pups were studied in eastern McMurdo Sound, Antarctica. Pups were marked and their apparent survival estimated by mark–recapture methods. The resulting estimates were used together with published estimates of adult survival, yearly sighting probabilities, and direct counts of pup production to simulate the dynamics of the population and evaluate the assumption that it is closed to immigration. Estimates derived from census data in 1982 and 1983 were over five times larger than those simulated. This discrepancy was due to the extremely low juvenile survival rates calculated from marked seals. Since few animals born in Erebus Bay return to breed, the large adult breeding population must be the result of substantial immigration, indicating an important role for juvenile dispersal in the population dynamics of Weddell seals.

Ethylene as an Indicator of Salt Tolerance in Rice1

Salt accumulation in cultivated and cultivable rice (Oryza sativa L.) and other croplands poses a serious threat to irrigated agriculture. Development of cultivars with high tolerance to salinity, and methodology to monitor and rapidly screen for salt tolerance may ameliorate the problem to a considerable extent. Because of the involvement of ethylene in growth responses under stress, its potential as an indicator of salt tolerance in rice in laboratory tests was investigated. Little ethylene was detected in rice seeds in the presence or absence of 0.1 M NaCl with up to 6 days of soaking. When 1‐aminocyclopropane‐1‐carboxylic acid (ACC) was used alone or in combination with salt, ethylene production began on the third day of soaking, reached a maximum by 4 to 5 days, and then declined. A parallel increase in ethylene production and shoot growth occurred in the presence or absence of salt with an increase in ACC concentration, reaching a maximum at 1 to 2 mM; at 5 to 50 mM both processes were inhibited. The ACC‐derived ethylene production and shoot growth were inhibited to varying extent in the rice genotypes by the addition of 0.1 M NaCl. At the saturating dose of ACC (2 mM), ethylene production and shoot growth appeared to depend on the activity of the ethylene forming enzyme (EFE) since Co2+, and not aminoethoxyvinylglycine (AVG), inhibited both processes. The capacity of rice to produce ACC‐dependent ethylene was a cultivar trait and correlated well (r = 0.91) with salt tolerance at the seedling stage. The data indicate that ethylene may serve as a biochemical marker for mass screening of large breeding populations of rice for salinity tolerance at the seedling establishment phase.

Selecting for resistance to diseases in early generations

Screening in the seedling and first clonal generations is a potentially cost-effective and efficient way to incorporate resistance to important diseases and reduce large breeding populations to manageable size. In practice, however, early screening is often limited by the need to maintain clean seedstocks of promising clones, unreliability of single-plant evaluations, associations of disease resistance with undesirable horticultural characteristics, or the higher priority placed on traits other than disease resistance. Possible methods to overcome these limitations are discussed. Examples of early generation screening for resistance to several diseases are given.

Pressure‐Volume Characteristics of Genotypes of Three Wheatgrass Species1

A need exists for drought tolerant forages that can produce acceptable yields on limited water. Techniques to accurately predict performance under drought stress are essential for selection of drought tolerant forages. Objectives of this study were to measure water relations of important forage species using the pressure volume (P‐V) method and to evaluate its usefulness predicting drought tolerance. The study was conducted using clonal selections of crested wheatgrass [Agropyron desertorum (Fisch.) Schult.], western wheatgrass (A. smithii Rydb.), and intermediate wheatgrass [A. intermedium (Link) Haloc.]. Pressure‐volume measurements were made using a pressure chamber on plants grown in both field and glasshouse environments. Data from the P‐V curves for initial turgor loss point, osmotic potential at full hydration and leaf water potential at zero pressure potential indicated differences among genotypes for all three species of wheatgrasses. Although only a limited number of genotypes were evaluated results appear more promising for selecting genotypes for drought tolerance in western than in crested or intermediate wheatgrasses. Species comparisons based on data from the P‐V relationship shows that genotypes of crested and western exhibit water relations that coincide with their greater drought tolerance. The P‐V technique requires a high labor input and will require modifications for practically evaluating large breeding populations for drought tolerance.

LONG-TERM RESPONSE TO SELECTION I. RELATION TO BREEDING POPULATION SIZE, INTENSITY, AND ACCURACY WITH ADDITIVE GENE ACTION

Dual CDC-6500 computers were used to simulate the probabilistic aspects of genetic selection and reproduction in random mating populations with additive gene action. These simulations involved either 10 or 20 replicates of 200 consecutive nonoverlapping generations for 72 combinations of breeding population sizes, mating ratios, selection intensities, and accuracies of genetic determination of quantitative phenotypes. The results demonstrate that at least some long-term responses can be characterized by modified exponential functions that, with increasing generations, approach asymptotically to limits whose expectations increase linearly with the inverse tangent of multiples of the expected initial responses. The multiplicative constants are greater for populations with large effective breeding population size than for those of smaller size. Agreement with and discrepancies from past theoretical results are discussed. The supposition is made that the general form of these equations will be retained for broader situations than those simulated, but probably not for nonadditive gene action.

Pollen collection from coconut flowers by means of a fluid-bed dryer

By using a portable laboratory fluid-bed dryer, it is possible to collect coconut pollen in the field, at rates of 55 to 60 g in 3 to 4 hours. Pollen collected this way has a viability similar to that of pollen from flowers dried for 40 hours on incubator shelves. It is suitable for immediate use or it may be stored. The technique opens new possibilities for hybrid seed production and for production of large breeding populations.

GENETIC VARIATION IN THE HORSESHOE CRAB (LIMULUS POLYPHEMUS), A PHYLOGENETIC "RELIC".

To many early evolutionists, particularly those of mutationist school, existence of evolutionarily stable forms (living fossils or phylogenetic relics) posed difficult problem, and, even among contemporary biologists, feeling that a long-standing stability of organization seems antithetical to concept of may persist (Volpe, 1967:140). In early part of this century, it was popularly believed that stable or slowly evolving (bradytelic) lines lacked genetic variation necessary for more rapid evolution as result of having abnormally low mutation rates; as Mayr (1963) puts it, lack of variation in time and space was interpreted as reflection of genotypic stability. Largely because of influence of Simpson (1944), Stebbins (1944), Schmalhausen (1949), and Dobzhansky (1951), mutationists' interpretation of has now been all but abandoned by evolutionists in favor of view that principal factors controlling rates of evolution are to be found in organism-environment relationship. Additionally, Mayr (1963 :304) has suggested that phenotypic uniformity in space and time is due to highly perfected 'buffering system,' in other words, to genetic homeostasis. The ecological hypothesis is well expressed in Simpson's (1944:141) conclusions that the final and probably most fundamental factor in relationship is that bradytelic groups are so well adapted to particular, continuously available environment that almost any mutation occurring in them must be disadvantageous and that bradytely results from equilibrium of large breeding populations of animals specifically adapted to continuously available environment that is relatively invariable . Yet, as Mayr (1963) notes, there is no evidence to refute (or to support) hypothesis that results from loss of mutability. The recent development of electrophoretic techniques for demonstrating allelic variation at loci controlling enzymes and other proteins in natural populations (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966) permits test of mutationists' thesis. The purpose of this study is to compare degrees of protein polymorphism and underlying genic heterozygosity in classic phylogenetic relic, horseshoe or king crab (Limulus polyphemus), and in several representatives of horotelic lines (those evolving at standard rates) for which data are already available. Evidence of level of genetic variation in Limulus similar to that in horotelic forms will support modern ecological and homeostatic interpretations of bradytely. But if mutationists' thesis is valid, Limulus should show an unusually low degree of polymorphism at its genetic loci. Limulus polyphemus is one of five living species of Subclass Xiphosura, group of marine arthropods usually placed with Subclass Eurypterida (extinct giant water scorpions) in Class Merostomata of Subphylum Chelicerata. Following period of adaptive radiation in early and middle Paleozoic, Merostomata became much less prominent after Permian. Several Mesozoic forms are known in Family Mesolimulidae, but by Tertiary fossil record is all but absent. There are presently three living genera, all belonging to Family Limulidae, only one of which, Tachypleus, is known from fossil record (Stormer,

The introduction of genetic material from inferior into superior strains.

1. A series of experiments have been carried out withDrosophila melanogasterin order to improve further a population, highly selected for a quantitative character, by introducing genetic material from an inferior population. The effects of selecting the latter before crossing to the selected line, of waiting after crossing before restarting selection and of varying the intensity of selection after crossing have been studied. The inferior population used was the large random breeding population from which the selected line had been produced by selecting downwards for sternopleural bristles.2. There proved to be some incompatibility between the conditions necessary to achieve the two criteria of success—the time to surpass the selected line and the extent by which it was eventually exceeded. The lines which surpassed it soonest had not been selected before crossing and had been selected intensely immediately afterwards. Those which surpassed it most had had the greatest amount of selection before crossing followed by the most intense selection afterwards.3. Crossing-over did not appear to be a limiting factor after crossing. Neither a period of relaxation after crossing nor a lower intensity of selection after crossing increased the chance of success.4. Genetic analyses were made of several lines which had exceeded the original selected line. In separate lines, second, third and fourth chromosomes were found which were superior to the chromosomes of the selected line. This and other evidence shows that the limits in such lines with a small number of parents are not in any way absolute but artefacts of the selection programme. The extreme third and fourth chromosomes were both recessive to that of the selected line in the effect on bristle score.5. A theoretical discussion of such selection programmes is given, assuming that the character is controlled by independently segregating loci. It is shown that, if two alleles at different loci have an equal but small chance of fixation in the initial selection, one because it is rare and the other because, though frequent, it has little effect on the character, such a programme will tend to pick up the former rather than the latter. Of two such rare alleles with an equal initial chance of fixation, the one additive and the other recessive, the chance of fixation of the latter will be greater in a crossing programme.

Equilibrium conditions in polysomic inheritance for a panmictic population.

A general model of a large 2m-ploid breeding population, withr loci andsh alleles at thehth locus is considered. It is assumed that the population is bisexual, non-overlapping and breeds by random mating. The genotypic structure of the population is presented as a bilinear form in the gametic output vectors where the genotype distribution is in the matrix form. Using the concept of the segregation distribution, the genotype proportions in the (n+1)st generation are given. An equilibrium condition for random chromosome segregation is obtained in terms of gene frequencies.

GENETIC VARIABILITY IN KOREAN POPULATIONS OFDROSOPHILA MELANOGASTER

Concealed genetic variability has been found by many workers in various species of Drosophila. Much of this work is reviewed in Spencer (1947). Notable among other studies are those of Chino (1936a, 1936b and 1937) on Japanese populations of D. virilis and of Stone and his group (1957) on irradiated Pacific island populations of D. anannasae. In general it is clear that natural populations of many Drosophila species carry large amounts of heterozygus genetic variability, with marked quantitative and qualitative differences, however, between both intraand inter-specific populations. In addition, rates of change in genetic variability within populations have sometimes been reported as rapid (Dubinin, 1946), or slow (Ives, 1945). A primary step in speciation and organic evolution is the occurrence, dissemination and frequency distribution of mutations in natural populations. The quality and quantity of mutations which accumulates in space and time within a population or species plays a significant role in the evolutionary changes occurring in the species. Little is actually known about the mechanisms which control these factors in natural populations, although some progress has been made in elucidating concepts bearing on these mechanisms. Of particular use in this respect have been lethal mutations, whose detection is relatively simple and easy. The frequency of such mutants in a population is related at least theoretically to the breeding structure of the population and to the nature of its environment. If there are no heterozygous effects, the concentration of recessive lethals at equilibrium should be lower in small populations than in continuously large populations (Wright, Dobzhansky and Hovanitz, 1942; Dubinin, 1946). Further, their frequency should be lower in small populations, at comparable generations, even before equilibrium is reached (Prout, 1954). Lessening of inbreeding at any time in a population leads to an increase in frequency of recessive lethal, or partially lethal, genes in the population's gene pool. Hence the frequency of these mutations found in a sample of flies from a population is in a sense a yardstick for estimating comparative sizes of populations (Ives, 1945) excepting as a selection operates appreciably for (Mazing, quoted in Dubinin, 1946) or against (Wright et al., 1942; Stern et al., 1952; Cordeiro, 1952; Prout, 1952) such heterozygous mutations. There is increasing evidence of geographical differences in concentration of lethals and other genetic variability. Dubinin (1946) also reported periodical fluctuations in lethal frequency correlated with seasonal oscillations in population size, a phenomenon not confirmed by others (Wright et al., 1942; Ives, 1954; Goldschmidt et al., 1955). Further work is needed along this line. In a large breeding population gene frequencies are determined by counteracting, systematic evolutionary forcesmutation, selection and migration. In a small, island-type population, even in one subject to oscillations in climatic and ecological conditions, random changes in gene frequencies are also important, for they will sometimes by themselves cause fixation or elimination of certain alleles without respect to their selective value