Better stay together: pair bond duration increases individual fitness independent of age-related variation

Prolonged pair bonds have the potential to improve reproductive performance of socially monogamous animals by increasing pair familiarity and enhancing coordination and cooperation between pair members. However, this has proved very difficult to test robustly because of important confounds such as age and reproductive experience. Here, we address limitations of previous studies and provide a rigorous test of the mate familiarity effect in the socially monogamous blue-footed booby, Sula nebouxii, a long-lived marine bird with a high divorce rate. Taking advantage of a natural disassociation between age and pair bond duration in this species, and applying a novel analytical approach to a 24 year database, we found that those pairs which have been together for longer establish their clutches five weeks earlier in the season, hatch more of their eggs and produce 35% more fledglings, regardless of age and reproductive experience. Our results demonstrate that pair bond duration increases individual fitness and further suggest that synergistic effects between a male and female''s behaviour are likely to be involved in generating a mate familiarity effect. These findings help to explain the age- and experience-independent benefits of remating and their role in life-history evolution.     

A 13-YEAR POPULATION STUDY OF THE BLACK EAGLES IN THEMATOPOS,RHODESIA, 1964–1976

Gargett, V. 1977. A 13-year population study of the Black Eagles in the Matopos, Rhodesia, 1964–1976. Ostrich 48:17-27.

The resident black Eagle Aquila verreauxi population in approximately 620 km² of the Matopos, Rhodesia, underwent changes from 1971 and appeared less stable than in the first five years. Six new territories were established in the National Park; two territories in Tribal Trust Land were abandoned; one territory in the National Park vacant for five years and one in Tribal Land vacant for at least 11 years were reoccupied. breeding data covering 13 years confirm the five-year findings. In 652 pair-years 442 breeding attempts were made with 339 young eagles f edging: a reproductive success rate of 0,52 young per pair per year. On average 68 % of the population bred every year, each pair attempted breeding in two years out of three, and one attempt in four was unsuccessful. Annual and individual variations in breeding performance were considerable, from 46% to 89% of the population breeding, and from two successes in six attempts for one pair to 12 successes in 12 attempts for two pairs. Over 13 years the percentage of the population breeding annually followed the form of a sine curve with a ten year period.

Fourteen factors that might affect annual and/or individual performance are considered. Below average rainfall years were followed by above average breeding; and generally fewer clutches were laid early after heavy rainfall in the three months preceding laying. Clutches laid late were less successful than those laid early. Breeding was affected by the proximity and intervisibility of nests, the previous year's performance and intraspecific disturbance. Appropriation of nests by other species prevented breeding and no new pair bred while establishing or re-establishing its territory. Building a new nest was followed by laying in the same season only if the nest was completed by mid-May, the peak laying period, building or partly building two nests in one season did not prevent breeding in the following year. At least 76% of clutches were c/2. The causes of two eaglets' deaths were ascertained and the remains of two adults were found. Pairs with territories in Tribal Trust Land had a significantly lower breeding performance than pairs in the protected National Park. Only traces of pesticide residues were found in four eggs. Observers' visits did not affect reproductive success.     

Effects of elevated CO2, elevated O3 and potassium deficiency on Norway spruce [Picea abies (L) Karst.]: seasonal changes in photosynthesis and non-structural carbohydrate content

Two clones of 5-year-old Norway spruce [Picea abies (L.) Karst.] were exposed to two atmospheric concentrations of CO2 (350 and 750 μmol mol^?1) and O₃ (20 and 75nmolmol^?1) in a phytotron at the GSF-Forschung-szentrum (Munich) over the course of a single season (April to October). The phytotron was programmed to recreate an artificial climate similar to that at a high elevation site in the Inner Bavarian Forest, and trees were grown in large containers of forest soil fertilized to achieve contrasting levels of potassium nutrition, designated well-fertilized or K-deficient. Measurements of the rate of net CO₂ assimilation were made on individual needle year age classes over the course of the season, chlorophyll fluorescence kinetics were recorded after approximately 23 weeks, and seasonal changes in non-structural carbohydrate composition of the current year's foliage were monitored. Ozone was found to have contrasting effects on the rate of net CO₂ assimilation in different needle age classes. After c. 5 months of fumigation, elevated O₃ increased (by 33%) the rate of photosynthesis in the current year's needles. However, O3 depressed (by 30%) the photo-synthetic rate of the previous year's needles throughout the period of exposure. Chlorophyll fluorescence measurements indicated that changes in photosystem II electron transport played no significant role in the effects of O₃ on photosynthesis. The reasons for the contrasting effects of O₃ on needles of different ages are discussed in the light of other recent findings. Although O₃ enhanced the rate at which CO₂ was fixed in the current year's foliage, this was not reflected in increases in the non-structural carbohydrate content of the needles. The transfer of ambient CO₂-grown trees to a CO₂-enriched atmosphere resulted in marked stimulation in the photosynthetic rate of current and previous year's foliage. However, following expansion of the current year's growth, the photosynthetic rate of the previous year's foliage declined. The extent of photosynthetic adjustment in response to prolonged exposure to elevated CO₂ depended upon the clone, providing evidence of intraspecific variation in the long-term response of photosynthesis to elevated CO₂. The increase in photosynthesis induced by CO₂ enrichment was associated with increased foliar concentrations of glucose, fructose and starch (but no change in sucrose) in the new growth. CO₂ enrichment significantly enhanced the photosynthetic rate of K-deficient needles, but there was a strong CO₂soil interaction in the current year's needles, indicating that the long-term response of trees to a high CO₂ environment may depend on soil fertility. Although the rate of photosynthesis and non-structural carbohydrate content of the new needles were increased in O₃-treated plants grown at higher levels of CO₂, there was no evidence that elevated CO₂ provided additional protection against O₃ damage. Simultaneous exposure to elevated O₃ modified the effects of elevated CO₂ on needle photosynthesis and non-structural carbohydrate content, emphasizing the need to take into account not only soil nutrient status but also the impact of concurrent increases in photochemical oxidant pollution in any serious consideration of the effects of climate change on plant production.     

Relating annual increments of the endangered Blanding's turtle plastron growth to climate

This research is the first published study to report a relationship between climate variables and plastron growth increments of turtles, in this case the endangered Nova Scotia Blanding's turtle (Emydoidea blandingii). We used techniques and software common to the discipline of dendrochronology to successfully cross‐date our growth increment data series, to detrend and average our series of 80 immature Blanding's turtles into one common chronology, and to seek correlations between the chronology and environmental temperature and precipitation variables. Our cross‐dated chronology had a series intercorrelation of 0.441 (above 99% confidence interval), an average mean sensitivity of 0.293, and an average unfiltered autocorrelation of 0.377. Our master chronology represented increments from 1975 to 2007 (33 years), with index values ranging from a low of 0.688 in 2006 to a high of 1.303 in 1977. Univariate climate response function analysis on mean monthly air temperature and precipitation values revealed a positive correlation with the previous year's May temperature and current year's August temperature; a negative correlation with the previous year's October temperature; and no significant correlation with precipitation. These techniques for determining growth increment response to environmental variables should be applicable to other turtle species and merit further exploration.     

SURVIVAL,PAIR BOND RETENTION AND NEST-SITE TENACITY IN BULLER'S MOLLYMAWK*

Adult Buller's Mollymawks Diomedea bulleri banded in 1948 and 1961 at the Snares Islands south of New Zealand have been recovered between 1961 and 1971. Survival curves are presented which indicate the existence of a constant annual mortality rate of not more than 11.1 % with a mean expectation of further life at the time of banding of not less than 8.5 years. Breeding birds show a strong tendency to retain both the pair bond and the nest-site. One pair was still breeding in 1971 at the same nest at which it bred 23 years earlier. No re-matings were recorded while the original partner was known to be still alive. The dimensions of eggs laid over the years by particular females are presented. These enable some general indications to be given of the effects of the age of the female on the dimensions of the eggs which she lays.     

The effects of enhanced ozone and enhanced carbon dioxide concentrations on biomass, pigments and antioxidative enzymes in spruce needles (Picea abies L.)

During one growing period, 5-year-old spruce trees (Picea abies L., Karst.) were exposed in environmental chambers to elevated concentrations of carbon dioxide (750 cm³ m^?3) and ozone (008 cm³ m^?3) as single variables or in combination. Control concentrations of the gases were 350cm³ m^?3CO₂ and 0.02 cm³ m ^?3 ozone. To investigate whether an elevated CO₂ concentration can prevent adverse ozone effects by reducing oxidative stress, the activities of the protective enzymes superoxide dismutase, catalase and peroxidase were determined. Furthermore, shoot biomass, pigment and protein contents of two needle age classes were investigated. Ozone caused pigment reduction and visible injury in the previous year's needles and growth reduction in the current year's shoots. In the presence of elevated concentrations of ozone and CO₂, growth reduction in the current year's shoots was prevented, but emergence of visible damage in the previous year's needles was only delayed and pigment reduction was still found. Elevated concentrations of ozone or CO₂ as single variables caused a significant reduction in the activities of superoxide dismutase and catalase in the current year's needles. Minimum activities of superoxide dismutase and catalase and decreased peroxidase activities were found in both needle age classes from spruce trees grown at enhanced concentrations of both CO₂ and ozone. These results suggest a reduced tolerance to oxidative stress in spruce trees under conditions of elevated concentrations of both CO₂ and ozone.     

Medical audit data: counting is not enough.

OBJECTIVE--To assess the meaningfulness of a year''s worth of audit data relating to all the inpatients of one consultant general surgeon and to question the usefulness of certain outcome measures. DESIGN--Analysis of records entered on to audit computer (Dunnfile) and relating to inpatient episodes for one consultant general surgeon over one year. Data obtained were compared with ward records and the patient administration system to check their accuracy. SETTING--The three hospitals and 12 wards in Brighton health district where the surgeon admitted patients. SUBJECTS--859 Records relating to inpatient episodes from 1 January to 31 December 1988. These covered 655 main procedures and 79 secondary procedures performed at the same time. MAIN OUTCOME MEASURES--Procedures were analysed by complexity of operation (BUPA code) and grade of surgeon; complications were counted and rates constructed by surgeon and by BUPA code: returns to theatre were analysed. RESULTS--Simple counts revealed some data, such as the fact that one registrar performed more major operations (32) than the senior registrars (22 and 14), and an analysis of complications showed that he had a lower complication rate (11.4% v 20.0% and 19.4%). But the simple complication rate disclosed nothing about whether the complication was avoidable. Likewise, the number of returns to theatre needed further qualification. Analysis of data collection for February to April 1988 showed a 30% deficit of information on the audit system compared with ward records and prompted a re-examination of everyone''s role in collecting data. After the year''s audit there was still a 17% shortfall compared with the district''s patient administration system, though some of this was accounted for by a backlog of work. CONCLUSIONS--It is difficult to ensure adequate data collection and entails everyone in an unfamiliar discipline. Connecting the audit system to the patient administration system would help. Despite the limitations of crude analyses of workload and complications rates, the audit data helped to measure activity and in the management of the firm. Nevertheless, time and care have to be taken in presenting and interpreting audit data carefully. IMPLICATIONS--Counting is not enough.     

Factors influencing pairbond stability in convict cichlids (Cichlasoma nigrofasciatum)

In isolated pairs of the biparental convict cichlid (Cichlasoma nigrofasciatum) caring for fry, pairbonds can be broken by removal of the fry. Within three hours, the mates of many pairs become so mutually aggressive that the female finally flees from her larger mate, who chases her. The pairbond is mended within seconds upon re-introduction of the fry. Yet removal of young leaves some pairs little or not at all affected. They seem to have more stable pairbonds. Multiple regression of data from 39 pairs on a quantitative measure of pairbond destruction revealed support for the so-called parity-hypothesis, which holds that a cichlid pair is compatible only when the female's aggressiveness compensates for her smaller size. The size of the female relative to the male's, and her aggressiveness relative to his in the undisturbed situation, proved positive predictors of pair stability in the fry stage. Thus the same factors which have earlier been shown to favour pair formation in other biparental cichlids seem also responsible for pair stability in a later stage of the reproductive cycle.     

Factors influencing reproductive success and litter size in captive island foxes

A severe decline of island foxes (Urocyon littoralis) on the northern Channel Islands in the 1990s prompted the National Park Service to begin a captive breeding program to increase their numbers. Using detailed records of all the fox pairs (N = 267) that were part of the program on San Miguel, Santa Rosa, and Santa Cruz Islands from its inception in 2000 through 2007, we identified factors influencing the breeding success of pairs in captivity in the interest of formulating strategies that could increase captive productivity. We compiled a database of variables including litter size, reproductive success, distance to nearest occupied pen during the breeding season, subspecies, exposure, female age, male age, age difference, female and male origin (wild vs. captive born), same versus different origin, years paired, previous reproductive success by the pair, previous reproductive success by the female, mate aggression-related injuries, male previous involvement in a pair with mate aggression, and female previous involvement in a pair with mate aggression. We used multiple linear regression to identify factors predictive of litter size, and logistic regression to predict the probability of reproductive success. A larger inter-pen distance, older male age, less exposure, and a smaller intra-pair age difference positively affected litter size. The probabilities of reproductive success increased with fewer years paired and less exposure. Comparatively, pairs with wild born females (vs. captive born females), and previously successful pairs (vs. previously unsuccessful and new pairs) were most likely to be successful. These results indicate that the optimal situation was to pair wild-caught females with older males in sheltered pens that were as far from other pens as possible, to maintain successful pairs and repair unsuccessful ones. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Female resistance and male force: context and patterns of copulation in the New Zealand stitchbird Notiomystis cincta

The New Zealand stitchbird or hihi Notiomystis cincta is unique in that it has two distinct mating positions, in addition to the male standing on the female's back, as is seen in all other birds, it also copulates face‐to‐face. In this study, 43 male stitchbirds first attracted a female to their territory and supplemented their within‐pair matings by intruding into other territories and attempting forced copulations – often resulting in high levels of female harassment. I recorded the temporal variation of both attempted and successful copulations relative to the female's fertile period in order to understand the function of copulation variation in this species. Each of 105 observed copulations were classified according to whether they were: (1) within‐pair or extra‐pair, (2) forced or unforced, and (3) face‐to‐face or standing. Two copulation categories ‘within‐pair unforced standing’ (50%) and ‘extra‐pair forced face‐to‐face’ (27%) accounted for the majority of all observed copulations, and this supported the previous categorisation of face‐to‐face copulation being forced by extra‐pair males in this species. However, in 10% of copulations the female conceded to copulate with an extra‐pair male in a standing position while displaying only subtle signs of resistance, thus, female stitchbirds may show convenience polyandry when the costs of resistance are high. The peak in copulation frequency centred on two days prior to the laying of the first egg and occurred within a period of six days before and seven days after the first egg was laid. Unsuccessful extra‐pair forced copulation attempts closely followed the distribution of all copulations. Male age and morphometrics did not predict whether a female would resist or accept extra‐pair copulation attempts, and female resistance did not appear to depend on male quality. Despite the current trend focussing on female fitness benefits associated with EPCs, it appears that male stitchbirds gain EPCs through forced copulation and females gain no apparent benefit.     

Lamb production using superovulation, embryo bisection, and transfer

In this study, 39 embryos from donor ewes superovulated with follicle stimulating hormone-pituitary (FSH-P) were bisected to produce pairs of monozygotic twin lambs for experimentation. Each pair obtained by bisecting 8-, 9- or 10-day-old embryos was immediately transferred surgically into a recipient ewe at the same physiological stage. Of the 39 recipients which received a pair of half-embryos by transfer into the uterine horn ipsilateral to the corpus luteum, 28 (72%) lambed. Eighteen of 28 recipients lambing (64%) produced pairs, i.e., 7 male and 11 female pairs. Ten of 28 lambings produced a single lamb, i.e., six males and four females. Overall yield (the number of lambs produced in relation to the number of embryos used) was 118%. This percentage tended to increase, depending on the day of collection (Day 8, 100%; Day 9, 118%; and Day 10, 131%).     

DIET,HOME RANGE,HUNTING AND REPRODUCTIVE BEHAVIOUR OF A PAIR OF DICKINSON'S KESTREL FALCO DZCKZNSONZ IN THE KRUGER NATIONAL PARK,SOUTH AFRICA

Summary

BENN, G.A. &; KEMP, A.C. 1995. Diet, home range, hunting and reproductive behaviour of a pair of Dickinson's Kestrel Falco dickinsoni in the Kruger National Park, South Africa. Ostrich 66: 81–91.

During July-December 1992, the diet, home range, hunting and reproductive behaviour of a pair of Dickinson's Kestrel Falco dickinsoni was recorded in the Kruger National Park, South Africa. Numerically, for both sexes combined, invertebrates formed the majority (56%) of the diet, while separately the female caught 75% and the male 49% invertebrate prey. During courtship and incubation the male supplied the female with primarily vertebrate prey and both provisioned mainly vertebrates to the nestlings (male = 80%; female = 57%). The non-breeding home range of the female was 27.8 km², and the breeding home range of the male was 26.3 km2. Both utilised their home ranges differentially, the area within a 2 km radius of the nest (12.6 km²) being used proportionally more than the remaining area. The home range of the female was compared to that of other Falco spp. and was larger than would be expected based on body weight. Perch-hunting was the only technique utilised by both sexes, with 79–80% of observed strike attempts from dead trees. During the day, the 9.emale spent 87% and the male 77% of the time perch-hunting, with respective hunting success rates of 69% and 58%. During courtship, the female spent much of her time (94%) close to the nest, where the male supplied her with prey. During incubation, the male spent 95% of his time within 2 km of the nest tree, where he hunted to supply the female with prey at a rate of 0.3 items.hr^?1 and assisted in nest defence. On occasion the male entered the nest to relieve the female, and remained in the cavity on average for 134 min (n = 5). As the young got older, the female spent less time at the nest and provisioned more items to the nestlings. Overall, there was an increase in the rate of prey provisioning to the nestlings from 0.45 items.hr^?1 (10 days old) to 0.85 items.hr^?1 (21 days old). The male initially passed prey to the female but provisioned directly to older nestlings.     

THE SOCIAL ORGANISATION OF BEE-EATERS (MEROPIDAE) AND CO-OPERATIVE BREEDING IN HOT-CLIMATE BIRDS

In the colony-nesting bee-eater Merops bulocki of the northern savannas of Africa males preponderate in the proportion 1 -5 to 1 female, initial breeding occurs at varying ages, and adults of both sexes and all age classes, particularly one-year-old males, assist breeding pairs with nest excavation, incubation, and provisioning the pulli and fledglings. Up to three helpers can occur at one nest, and they may be siblings of a previous year's brood. Pair-bonds are for life, and bonds formed between the breeding pair and their helpers may endure until next season. Some evidence suggests that other gregarious open-country bee-eater species are similar in these respects but that forest and solitary-breeding savanna species have the sex ratio at parity and do not breed co-operatively. Breeding dispersion of savanna species is related to specific size, the largest (which forage most widely) nesting in the densest aggregates. Other instances of birds with comparable social organisations are briefly reviewed. The 80 species involved comprise broad systematic and ecological spectra and about 60 of them inhabit hot climates in America, Africa and Australia. It is suggested that co-operative breeding provides a reserve of experienced individuals whose recruitment to breeding permits (1) fine control of stable populations in non-seasonal habitats (tropical forest) within the capacity of the food resources; (2) crude adjustment of fluctuating populations in habitats with unpredictable rainfall (Australia), enabling them rapidly to recuperate from depression and to achieve the highest population level the improved food resources can sustain; and (3) regulation of populations in intermediate habitats (tropical savannas) by either means according to the dictates of immediate ecological circumstances.     

Assortative Pairing Based on Contact Call Similarity in Budgerigars, Melopsittacus undulatus

A trait typical of parrots, but rare in other groups of birds, is the acquisition of new learned calls (acquired by copying conspecifics) throughout an individual's lifetime. The significance of this distinctive psittacid trait is not understood. In budgerigars, females preferentially affiliate with unfamiliar males whose contact calls resemble their own during brief dyadic choice trials; also, in forced‐pair situations, contact call similarity of members of pairs increases as a result of a male tendency to imitate his mate's call type. The functions of budgerigar call imitation and preference for pre‐pairing similarity are currently unknown. Moreover, as budgerigar pair formation occurs over a span of days or weeks, it is important to determine whether birds in breeding colonies assort and proceed to breed on the basis of pre‐pairing contact call similarity, and whether high levels of call similarity are maintained after pair formation is complete. To explore these questions, we recorded contact calls of male and female budgerigars before and after they were placed into an aviary equipped for breeding. As predicted, birds paired assortatively based on pre‐pairing call similarity. Once birds had paired, their calls converged further in acoustic structure, as previous work had led us to expect. However, after eggs were laid and the males began to feed their mates, the calls of mated birds diverged, suggesting that there might be some cost to maintenance of shared calls. Male care‐giving correlated with the degree to which his pre‐pairing calls resembled those of his mate, but not with the similarity achieved through convergence. These results suggest that female budgerigars may use a male's pre‐pairing call similarity as a predictor of paternal investment. The questions of why such similarity predicts male care‐giving, and why calls converge following initial pairing activities, require further work.     

The role of brain scanning in the differential diagnosis of seizures

Of 200 patients with seizures who had brain scans, the studies were abnormal in 86% of patients with tumours and in 67% of those with strokes. The scan detected 84% of the 19 potentially curable lesions. There were no false positive studies. Seventy-nine per cent of patients with seizures of less than one year''s duration had an abnormal scan; however, the scan was also abnormal in 22% of patients with non-focal seizures of less than one year''s duration.     

Extra‐pair paternity in Swainson's Hawks

ABSTRACT Although passerines have been relatively well studied and many species found to exhibit relatively high rates of extra‐pair paternity (EPP), less is known about the frequency of EPP in other avian taxa, including raptors. From 2008 to 2010, we examined the frequency of EPP in a population of Swainson's Hawks (Buteo swainsoni) in Butte Valley, California. We examined paternity of 56 nestlings from 19 pairs and 27 broods and found that only three nestlings (5%) in two (7%) broods were the result of extrapair fertilizations. This relatively low frequency of EPP may be the result of mechanisms that reduce the likelihood of extra‐pair fertilization (e.g., mate guarding and frequent copulation), or could result from females limiting EPP to assure paternity of the social male and ensure paternal investment in offspring.     

Effects of Pair-Bond and Social Context on Male–Female Interactions in Captive Titi Monkeys (Callicebus moloch, Primates: Cebidae)

In monogamous species, an abiding relationship between a specific adult male and a specific adult female is a defining feature of the social system. The interactions between these individuals are influenced by many factors, including not only the history of their relationship (for example, development of a mutual bond), but also the immediate effects of the prevailing social context (for example, presence and sex of extra‐pair conspecifics). In this study we examined the effects of an existing bond and of social context on interactions between adult heterosexual pairs of the monogamous titi monkeys (Callicebus moloch). Twelve adult males and 12 adult females were tested with their cagemates and with an unfamiliar partner of the opposite sex in five social contexts: (1) mated male–female pair; (2) unfamiliar pair; (3) single female; (4) single male; and (5) empty stimulus cage. Results show that mated pairs were more affiliative than unfamiliar pairs and differentiated social contexts more sharply. Males were more responsive to context than females. Distance between mates was less and physical contact was more frequent in the presence of male–female pairs or a single male, than in the presence of a single female or an empty cage. These findings suggest that the presence and sex of strangers have a stronger influence on male–female interactions when the pair has an existing relationship.     

The effects of mate separation on pair re-formation in the Texas cichlid fish (Cichlasoma cyanoguttatum)

This study tested whether mated pairs of Texas cichlid ($\text{Cichlasoma cyanoquttatum}$) would reform and continue rearing their offspring after being separated for either one, four, or ten days. All pairs successfully re-formed after one day, while only 50% re-formed after 4 days. No pair re-formed after 10 days of separation. In successfully re-formed pairs, the female was always more aggressive than her mate, irrespective of whether the female was the resident or the returning individual. Aggressiveness seemed related to the female's attempt to control the male's parental behavior. For pairs which did not re-form, the resident parent, either the male or female, violently and continously attacked the returned mate.     

An Alternative Method for Evaluating Soybean Tolerance to Heterodera glycines in Field Plots

Alternate planting dates and periodic destruction of the previous year''s soybean crop as well as 1-year bare fallow were used to establish a range of population densities ofHeterodera glycines for the subsequent year. Soybean cultivar Coker 156 (susceptible, moderately tolerant) was compared to cultivars Essex (susceptible, intolerant) and Bedford (resistant) to evaluate tolerance at different H. glycines population densities established through the previous year''s treatments. Yield of Coker 156 was consistently intermediate between yields of Bedford and Essex in 1986 and 1987. Yield of Essex was negatively correlated (P = 0.05) with preplant egg numbers of H. glycines in 1987, whereas yield of Bedford and Coker 156 were not related to nematode density. Reproduction of H. glycines was greater (P = 0.05) on the moderately tolerant Coker 156 than on either of the other cultivars.     

Chromosome-specific control of chiasma formation

A desynaptic mutant of Hypochoeris radicata, 2n = 8, has been found in a population from France. The mutant is remarkable in that at metaphase-I over 90% of PMC's have a pair of univalents while two pairs are found in only 1% of cells. Only chromosome IV is affected by the desynapsis which is controlled by a single recessive gene. Bivalent chiasma frequency in cells with a pair of univalents is higher than in cells with complete bivalent formation indicating partial dependence of cell chiasma number on the availability of some factor within the anther. The F₂, produced by full-sib mating, has a raised frequency of univalents in the chromosomes other than pair IV. The higher level results from enforced sib-mating of an obligate outbreeder and is not related to the action of the major gene. It is suggested that chiasma formation is controlled both polygenically and by major genes which operate on a hierarchical system. Some genes affect the behaviour of the entire complement while the action of others is specific to individual chromosomes.