A stereological method for estimating fish fecundity

This paper describes the application and evaluation of a stereological method for counting fish oocytes. The method has been applied to herring, Clupea harengus L., Dover sole, Solea solea L., and mackerel, Scomber scombrus L., and compared with existing volumetric and/or automated particle counter methods. The stereological method has been shown to given similar results and to have a number of distinct advantages over the other methods.     

THERMAL EVOLUTION OF EGG SIZE IN DROSOPHILA MELANOGASTER

We measured the size of eggs produced by populations of Drosophila melanogaster that had been collected along latitudinal gradients in different continents or that had undergone several years of culture at different temperatures in the laboratory. Australian and South American populations from higher latitudes produced larger eggs when all were compared at a standard temperature. Laboratory populations that had been evolving at 16.5°C produced larger eggs than populations that had evolved at 25°C or 29°C, suggesting that temperature may be an important selective agent in producing the latitudinal clines. Flies from laboratory populations produced larger eggs at an experimental temperature of 16.5°C than at 25°C, and there was no indication of genotype-environment interaction for egg size. Evolution of egg size in response to temperature cannot be accounted for by differences in adult body size between populations. It is not clear which life-history traits are direct targets of thermal selection and which are showing correlated responses, and disentangling these is a task for the future.     

Effect of temperature on development rate and intrinsic rate of increase ofAphis gossypii reared on greenhouse cucumbers

Thermal requirements for development and life table statistics ofAphis gossypii Glover (Homoptera, Aphididae) were determined over a range of constant temperatures from 10 to 30°C. The lower development threshold and the sum of effective temperatures were 6.9°C and 90.1°C, respectively, for preimaginal development, and 5.8°C and 113.6°C from birth to the onset of reproduction. Mean total fecundity ranged from 36 larvae per female at 10°C to 76 larvae at 30°C. On a time scale of days, net reproductive rate (R _o ) increased with increasing temperature while generation time (T) decreased causing the intrinsic rate of increase (r _m ) to increase linearly from 0.115 to 0.465. On a day-degree scaler _m only varied from 0.019 to 0.028 because the growth ofR _o was compensated by an increase inT with increase in temperature. The nearly constantr _m in terms of day-degrees, over a wide range of temperatures, greatly simplifies the prediction of future population numbers ofA. gossypii.     

The age, growth and reproduction of Chondrostoma polylepis willkommi in a seasonal stream in the Guadalquivir River basin (southern Spain)

The age, growth and reproduction of Chondrostoma polylepis willkommi , an endemic cyprinid from the Iberian Peninsula, was studied for 2 years in a seasonal stream in the Guadalquivir River basin. Annual growth was low. Maximum ages observed were 5+ in males and 7+ in females. There was no significant difference in growth between sexes. Seasonal growth period started in May and continued for 2 to 6 months depending on age. The mean lengths of 1 + and 2+ age-groups decreased once the growth period had finished, probably related to reproductive stress and adverse ecological conditions, and this could explain the occurrence of Rosa Lee's phenomenon. Both sexes matured at the end of their second year of life (1+). There was a significant difference from 1: 1 in the overall sex ratio of 334 males to 464 females. Reproductive period started in March and lasted until May. Ch. p. willkommi was a multiple spawner that released two batches of eggs per female each year. The regression between fecundity ( F ) and fork length (l., mm) was: F =6.20 10³ L.^2.78. In females from age 2 + onwards, relative annual investment in somatic growth and reproduction was equivalent, implying that the same energy was allocated to reproduction as to maintenance/growth. Compared with other Ch. polylepis populations, the life-history patterns of this population, located in a small and seasonal stream in a southern latitude, were characterized by a low annual growth, a trade-off between reproduction and growth/maintenance, early maturity, low number of age-groups and high fecundity from multiple spawnings.     

The effect of a foliar disease (rust) on the development of Gastrophysa viridula (Coleoptera: Chrysomelidae)

Abstract.

• 1 Gastrophysa viridula Degeer (Coleoptera: Chrysomelidae) and the pathogenic rust fungus Uromyces rumicis (Schum.) Wint. both occur on leaves of Rumex crispus L. and R.obtusifolius L. Individual stages of beetle development, and egg laying, were compared on healthy and infected leaves of each plant species in the laboratory. Oviposition choice was investigated in the field and laboratory.
• 2 Beetles reared on infected leaves of each species had greater larval mortality and slower development than those reared on healthy leaves. Although larvae feeding on infected leaves consumed up to 2.5 times more dry weight than those reared on healthy leaves, they had a lower relative growth rate and pupated at a lower weight. These changes were consistent with the reduced nutritive quality of rust-infected Rumex leaves.
• 3 Fecundity of beetles reared on infected leaves of both species was considerably reduced. Eggs laid by beetles feeding on infected R.crispus leaves also had a reduced viability.
• 4 The beetle developed consistently poorer on healthy R.crispus than on healthy R.obtusifolius throughout its life-cycle. Differences in larval performance were greater between host species than between infected and healthy leaves.
• 5 Oviposition was similar on infected and healthy R.crispus in both the laboratory and field. However, adults consumed less, and laid fewer eggs on infected than on healthy R.obtusifolius. The pattern of egg laying on different aged leaves was affected by rust infection: a greater proportion of eggs was laid on the older, infected leaves, than on the equivalent aged leaves on the healthy plants. Few larvae survived from eggs laid on rusted leaves in the field.

     

THE POPULATION DYNAMICS OF NORTHERN SEA LIONS, 1975-1985

Abstract: Populations of northern sea lions ( Eumetopias jubatus ) in the vicinity of Marmot Island, Alaska declined during 1975–1985 at about 5% per year (Merrick et al. 1987). The cause of this decline is not known. A life table for the northern sea lion was calculated assuming that life spans follow a Weibull distribution. Samples of northern sea lions taken in the vicinity of Marmot Island, Alaska during 1975–1978 and 1985–1986 indicate that the average age of females older than 3 yr increased about 1.55 yr (SD = 0.35 yr) while the population was declining at about 5% per year. Fecundity rates decreased by 10% over the same period, but the decrease was not statistically significant (Calkins and Goodwin 1988). Possible causes of the population decline and the change in age structure were examined by writing the Leslie matrix population equation in terms of changes in juvenile and adult survival rates and fecundity, and examining the short–term behavior of the trajectories of the average age of adult females, total number of females, and total number of pups with respect to those changes in the vital parameters. From the observed rate of declines of adults and the changes in average age of adult females and fecundity, estimates of the changes in adult and juvenile survival were calculated; estimates of the standard deviations of these changes were estimated via a bootstrap procedure. One purpose of this exercise is to aid in setting priorities for research for determining the cause of the decline. An explanation for the observed declines in numbers of adult sea lions consistent with the observed fecundity rates, a rate of decrease of 5% in the number of adults, and the corresponding increase in average age (of females age 3 yr and older) was a 10%–20% decrease in the survival of juveniles (age 0-3 yr) coupled with an insignificant change in adult survival (0.03%, SD = 1%).     

Pollen Production in the Androdioecious Datisca glomerata (Datiscaceae): Implications for Breeding System Equilibrium

Abstract Androdioecy (the presence of males and hermaphrodites in a breeding population) is a rare reproductive system in plants, with Datisca glomerata (Datiscaceae) representing the only well-documented example. Recent reports of high outcrossing rates, inbreeding depression, and high male pollen production satisfy theoretical predictions for the continued maintenance of androdioecy in populations of this species. However, in prior studies pollen production was measured indirectly in terms of numbers of anthers per flower—based on the assumption that male and hermaphroditic plant have equal numbers of flowers and that anthers from the two sexual morphs produce equivalent amounts of pollen. Herein, we demonstrate that male and hermaphrodite plants do not differ significantly in terms of flower number, but that pollen production in anthers from hermaphroditic plants is 12.6% higher than in anthers from male plants, thus refining the estimate of relative pollen fecundity of male versus hermaphrodite plants. The differential lowers the frequency of males predicted by theory, but is still consistent with the maintenance of androdioecy in this species.     

The fecundity patterns of S and L type rotifers of Brachionus plicatilis

Fecundity patterns of S and L type rotifers Brachionus plicatilis, which were previously found to have distinct growth ability, were analyzed using data of age-specific fecundity. The data were obtained by individual cultures of S and L type strains at 17, 24 and 34 °C. The pattern was analyzed by using normal probability functions. When the age was transformed into logarithmic value, the S and L types had an identical pattern of fecundity at every temperature. This fact indicates that the difference of the growth response to the temperature between S and L type strains stemmed only from the differences in net reproduction rate, not from the pattern.     

Effects of host instar and size on parasitization efficiency and life history parameters ofOpius dissitus

The braconid parasitoidOpius dissitus Muesebeck (Hymenoptera, Braconidae) produced 1.7 to 3 times more offspring when provided second and third instar leafminers (Liriomyza sativae Blanchard) as compared to first instars. Females arising from parasitization of different instars did not differ significantly in numbers of chorionated eggs in their ovaries at adult eclosion. Development time was prolonged by about two days when parasitoid oviposition occurred in first, as compared to third instar hosts. Parasitoid length was positively correlated with host weight (r²=0.75). Because only 7% of variation in host weight could be explained by host density, parasitoid length varied considerably among hosts reared at the same density. Longevity and lifetime fecundity of parasitoids were inversely related to the weight of their hosts.