INTENSITY DISCRIMINATION IN THE HUMAN EYE : II. THE RELATION BETWEENΔI/IAND INTENSITY FOR DIFFERENT PARTS OF THE SPECTRUM

1. A new apparatus is described for measuring visual intensity discrimination over a large range of intensities, with white light and with selected portions of the spectrum. With it measurements were made of the intensity ΔI which is just perceptible when it is added for a short time to a portion of a field of intensity I to which the eye has been adapted. 2. For white and for all colors the fraction ΔI/I decreases as I increases and reaches an asymptotic minimum value at high values of I. In addition, with white light the relation between ΔI/I and I shows two sections, one at low intensities and the other at high intensities, the two being separated by an abrupt transition. These findings are contrary to the generally accepted measurements of Koenig and Brodhun; however, they confirm the recent work of Steinhardt, as well as the older work of Blanchard and of Aubert. The abrupt transition is in keeping with the Duplicity theory which attributes the two sections to the functions of the rods and cones respectively. 3. Measurements with five parts of the spectrum amplify these relationships in terms of the different spectral sensibilities of the rods and cones. With extreme red light the relation of ΔI/I to I shows only a high intensity section corresponding to cone function, while with other colors the low intensity rod section appears and increases in extent as the light used moves toward the violet end of the spectrum. 4. Like most of the previously published data from various sources, the present numerical data are all described with precision by the theory which supposes that intensity discrimination is determined by the initial photochemical and chemical events in the rods and cones.     

Evidence for homologous pairing of chromosomes prior to meiotic prophase in maize

Relative positions of homologous heterochromatic regions of maize chromosomes were studied at premeiotic interphase, at tapetal mitotic interphase and at root tip mitotic interphase. In all three kinds of cell homologues were found to be situated significantly nearer to each other than to heterologues. It is concluded that some degree of homologous chromosome pairing may occur widely at anaphase or telophase (where it is easily overlooked) and that therefore, as has been previously suggested, homologues may be loosely aligned throughout premeiotic interphase in preparation for their subsequent synapsis.     

Genetic improvement of a parasitoid: Response of Trioxys pallidus to laboratory selection with azinphosmethyl

Colonies of Trioxys pallidus Haliday (Hymenoptera: Aphidiidae) were collected from California walnut orchards during 1985–86 and screened for variability of responses to azinphosmethyl. Variability was found and laboratory selection was initiated with four colonies. All colonies responded to selection; after seven to 12 selections, three colonies were combined (Select colony) and selection was conducted an additional 27 times. The corresponding base colonies were also combined and maintained for comparisons (Base). Concentration/mortality lines obtained for the Select and Base colonies after five and 27 selections indicated seven‐ and 9.2‐fold differences in LC₅₀ values respectively. Survival of the resistant strain varied with bioassay method and decreased in the order: clip cages on treated foliage > treated plastic cups with untreated mesh tops > treated plastic cups with treated mesh tops> treated cups with treated solid lids. The initial bioassay results with treated plastic cups suggested that the selection response was inadequate to allow the resistant strain to survive field rats of azinphosmethyl. When the resistant strain's survival was evaluated with bioassay methods more closely mimicking field conditions (clip cages on field‐treated foliage), we concluded the selected strain could survive field rates of azinphosmethyl.     

Ten families with fragile X syndrome: linkage relationships with four DNA probes from distal Xq

Summary We present clinical, cytogenetic, and linkage data of four DNA probes from the terminal long arm of the X chromosome in ten new families with fragile X syndrome. A prior/posterior method of multipoint linkage analysis is employed to combine these results with published data to refine the linkage map of terminal Xq. Ten possible probe/disease orderings were tested. The order with the greatest posterior probability (0.78) of the five loci is 52a-F9-fragile X gene-DX13-St14, although the order with reversal of the positions of 52a and F9 has a posterior probability 0.15. The mean estimates of the distances between the probes and the fragile X gene are 38cM and 33cM for the proximal probes 52a and F9, and 8 cM and 12 cM for the distal probes DX13 and St14. Although the current method of choice in the prenatal diagnosis and carrier detection of the fragile X syndrome remains detailed cytogenetic analysis, consideration is given to the potential role of these DNA probes, both singly and in pairs.     

HOST-RANGE,PLAQUE-MORPHOLOGY STUDIES OF CYANOPHAGE LPP-11

Transduction by temperate cyanophage plays an important role in understanding the effects of environmental pollution on genetic function. Using a new isolate, this paper illustrates the influence of contaminants and the rapid variations that result as a virus particle passes through successive hosts. Host-range and plaque-morphology, using an extended range of genetically-differing hosts, compares archetype LPP-1 cyanophage cultured on microbially contaminated hosts with bacteria-free cyanophage cultured on pure host strains. Microbial contamination can alter the host-range and serology of the cyanophage produced. It is suggested that bacteria are involved in the virus infection of cyanophycean hosts and that the study of host-range and plaque-morphology can aid in the biological characterization and segregation of mutants illustrating mechanisms of inter generic transfer of genetic material. It is shown that derivatives of archetype LPP-1, cultured on axenic hosts, possess a host-range, plaque-morphology and serology similar or identical to that of the temperate cyanophage, S3.     

Further studies on pachytene pairing failure in maize

Summary Pairing failure in pachytene chromosomes was studied against a genetically constant background. Very significant negative correlations were found between total chromosome length per cell on the one hand and number of terminal pairing failures and their total extent (per cent length) on the other, and between number of terminal failures and their average extent (per cent length). No significant correlation was found, however, between total chromosome length per cell and the average extent (per cent length) of pairing failure. If the pairing failures are dissociations increasing in extent and number during the pachytene stage studied, the simplest reconciliation of the results requires that the average rate of their extension be roughly proportional to total chromosome length or that certain constants be related to each other as described. An alternate interpretation, that the pairing failures were present before pachytene and that chromosomes are shortening faster in cells containing them, is favored by the findings that heterogeneity is low between anthers in number of failures per cell and that no significant correlation was found between anthers in total chromosome length and number of pairing failures. The possibility is also discussed that the pairing failures are a complex combination of both initial synaptic failure and later dissociation.Distributions of chromosomes containing pairing failure at both ends and of those containing both terminal and intercalary pairing failures generally follow expectations of randomness.This work was supported by National Institute of Health Grant Number RA-6492 and by National Science Foundation Grant Number G 7068.     

The pattern of pairing that is effective for crossing over in complex B-A chromosome rearrangements in maize

Special stocks involving complex B and A doubly translocated and recombined chromosomes were utilized to study the frequency and distribution of chiasmata under constrained conditions. The studies allow comparisons of frequency of pairing effective for crossing over in segments of different length and location in chromosomes that are present in disomic and trisomic quantity. Results provide new evidence for independent initiation of effective pairing in intercalary chromosome regions and suggest sequential events in the establishment of effective pairing, some of which may depend upon synaptic extension or two-by-two prealignment. Pairing frequency may depend directly on segment length under potentially competitive conditions. Evidence was not found for heterogeneity of pairing capacity within the regions studied.