Integration of light and temperature sensing by liquid-liquid phase separation of phytochrome B

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Two components of the receptor current are developed from distinct elementary signals in Limulus ventral nerve photoreceptor

Transient elementary currents, bumps, stimulated by short dim light flashes were measured in ventral nerve photoreceptors of Limulus. It is demonstrated that light activates two types of bumps, which form two distinct components of the receptor current at higher light intensities. The two bump types, which are both assumed to be activated by single absorbed photons, differ in current amplitude and kinetic parameters. The current amplitude of one bump type is smaller than 0.3 nA and that of the other type is in the usual current range of up to several nanoamperes. The average latency of small bumps measured from the short stimulus flash is shorter than that of the large bumps. The small bumps have slower activation kinetics than the large bumps. It is demonstrated that with increasing flash intensity the small bumps overlap first and form a macroscopic current, on top of which the large bumps are superimposed. Results indicate that a single absorbed photon selectively activates only one kind of the enzyme cascades evoking one bump type. We conclude that the active meta conformation of a rhodopsin molecule selectively binds a specific type of G-protein, which is involved in the stimulation of one of the transduction cascades. The two bump types, which are the elements of two macroscopic current components support the previous assumption that light activates different transduction mechanisms in Limulus photoreceptors.     

Dysregulation of Cav1.4 channels disrupts the maturation of photoreceptor synaptic ribbons in congenital stationary night blindness type 2

Mutations in the gene encoding Ca_v1.4, CACNA1F_, are associated with visual disorders including X-linked incomplete congenital stationary night blindness type 2 (CSNB2). In mice lacking Ca_v1.4 channels, there are defects in the development of “ribbon” synapses formed between photoreceptors (PRs) and second-order neurons. However, many CSNB2 mutations disrupt the function rather than expression of Ca_v1.4 channels. Whether defects in PR synapse development due to altered Ca_v1.4 function are common features contributing to the pathogenesis of CSNB2 is unknown. To resolve this issue, we profiled changes in the subcellular distribution of Ca_v1.4 channels and synapse morphology during development in wild-type (WT) mice and mouse models of CSNB2. Using Ca_v1.4-selective antibodies, we found that Ca_v1.4 channels associate with ribbon precursors early in development and are concentrated at both rod and cone PR synapses in the mature retina. In mouse models of CSNB2 in which the voltage-dependence of Ca_v1.4 activation is either enhanced (Ca_v1.4_I756T) or inhibited (CaBP4 KO), the initial stages of PR synaptic ribbon formation are largely unaffected. However, after postnatal day 13, many PR ribbons retain the immature morphology. This synaptic abnormality corresponds in severity to the defect in synaptic transmission in the adult mutant mice, suggesting that lack of sufficient mature synapses contributes to vision impairment in Ca_v1.4_I756T and CaBP4 KO mice. Our results demonstrate the importance of proper Ca_v1.4 function for efficient PR synapse maturation, and that dysregulation of Ca_v1.4 channels in CSNB2 may have synaptopathic consequences.     

Male Seychelles warblers use territory budding to maximize lifetime fitness in a saturated environment

In cooperatively breeding species, helping at the nest and buddingoff part of the natal territory have been advanced as strategiesto increase fitness in an environment that is saturated withterritories. The importance of helping or territory buddingas a determinant of lifetime reproductive success (LRS) hasbeen debated because the potential benefits of both strategiescould not be separated. Here we test the causes and the immediateand future fitness consequences of single dispersal decisionstaken by male Seychelles warblers (Acrocephalus sechellensis).Males breeding in high-quality territories (high food abundance)have significantly higher LRS than similar-aged males buddingoff part of the parental territory. Initially, budders havea low reproductive success (because of limited food resourcesor absence of a breeding partner). However, they have a longlife span and inherit high-quality territories through sitedominance, by which they gain higher LRS than breeders on low-qualityterritories, helpers, or floaters. Experimental creation ofmale breeding territory vacancies showed that most young malesbecame budders because of intense competition for high-quality territories. The translocation of warblers to the previouslyunoccupied Aride Island shows that males behave according tothe expected fitness benefits of each dispersal strategy. Inthe absence of competition for territories on Aride, all youngmales bred in high-quality territories. However, after saturationof high-quality habitat with territories, most males becamebudders rather than breeders on low-quality habitat, helpers,or floaters.     

The consequences of adult foraging success on the components of lifetime fitness in a semelparous,sit and wait predator

Summary We evaluated the role of adult foraging success in the lifetime fitness of female crab spidersMisumena vatia. Misumena are semelparous, sit and wait predators that hunt for insect prey on flowers, in this study primarily on inflorescences (umbels) of milkweedAsclepias syriaca. We used path analysis to integrate previously performed experimental and observational studies, thereby establishing the magnitude, correlations and causal relationships of key foraging and life history variables and their roles in lifetime fitness. A path proceeding from maternal hunting patch choice through maternal mass, clutch mass and number of dispersal-age young was the dominant element and explained a large part of the variation. Other paths that incorporated parasitism of the egg mass and predation of young leaving the nests made only small impacts on variation. No trade-offs were found, primarily because a single factor, maternal mass (a maternal effect) resulting from foraging success, provided major benefits for successive life history stages. Since differences in the numbers of eggs, egg loss and mortality at dispersal resulted almost entirely from differences in maternal mass, they are controlled by the maternal generation and, thus, are appropriately attributed to the lifetime fitnesses of the mothers, rather than to those of their offspring.     

Actions of Ca2+ on an Early Stage in Phototransduction Revealed by the Dynamic Fall in Ca2+ Concentration during the Bright Flash Response

To study the actions of Ca²⁺ on “early” stages of the transduction cascade, changes in cytoplasmic calcium concentration (Ca²⁺ _i) were opposed by manipulating Ca²⁺ fluxes across the rod outer segment membrane immediately following a bright flash. If the outer segment was exposed to 0 Ca²⁺/0 Na⁺ solution for a brief period immediately after the flash, then the period of response saturation was prolonged in comparison with that in Ringer solution. But if the exposure to 0 Ca²⁺/0 Na⁺ solution instead came before or was delayed until 1 s after the flash then it had little effect. The degree of response prolongation increased with the duration of the exposure to 0 Ca²⁺/0 Na⁺ solution, revealing a time constant of 0.49 ± 0.03 s. By the time the response begins to recover from saturation, Ca²⁺ _i seems likely to have fallen to a similar level in each case. Therefore the prolongation of the response when Ca²⁺ _i was prevented from changing immediately after the flash seems likely to reflect the abolition of actions of the usual dynamic fall in Ca²⁺ _i on an early stage in the transduction cascade at a site which is available for only a brief period after the flash. One possibility is that the observed time constant corresponds to the phosphorylation of photoisomerized rhodopsin.