Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum

The mixotrophic (bacterivorous), freshwater chrysophyte Dinobryon cylindricum was cultured under a variety of light regimes and in bacterized and axenic cultures to investigate the role of phototrophy and phagotrophy for the growth of this alga. D. cylindricum was found to be an obligate phototroph. The alga was unable to survive in continuous darkness even when cultures were supplemented with high concentrations of bacteria, and bacterivory ceased in cultures placed in the dark for a period longer than one day. Axenic growth of the alga was poor even in an optimal light regime. Live bacteria were required for sustained, vigorous growth of the alga in the light. Carbon (C), nitrogen (N), and phosphorus (P) budgets determined for the alga during growth in bacterized cultures indicated that bacterial biomass ingested by the alga may have contributed up to 25% of the organic carbon budget of the alga. Photosynthesis was the source of most (75%) of the organic carbon of the alga. D. cylindricum populations survived but did not grow when cultured in a continuous low light intensity (30 E m^–2 sec^–1), or in a light intensity of 150 E m^–2 sec^–1 for only two hours each day. Net efficiency of incorporation of bacterial C, N, and P into algal biomass under these two conditions was zero (i.e., no net algal population growth). We conclude that the primary function of bacterivorous behavior in D. cylindricum may be to provide essential growth factor(s) or major nutrients for photosynthetic growth, or to allow for the survival of individuals during periods of very low light intensity or short photoperiod. Offprint requests to: David A. Caron     

Eicosanoids mediate Manduca sexta cellular response to the fungal pathogen Beauveria bassiana: a role for the lipoxygenase pathway

Many studies have documented the involvement of eicosanoids in insect cellular immune responses to bacteria. The use of the fungal pathogen Beauveria bassiana as a nodulation elicitor, with inhibition of phospholipase A(2) by dexamethasone, extends the principle to fungi. This study also provides the first evidence of involvement of the lipoxygenase (LOX) pathway rather than the cyclooxygenase (COX) pathway in synthesis of the nodulation mediating eicosanoid(s). The LOX product, 5(S)-hydroperoxyeicosa-6E,8Z,11Z,14Z-tetraenoic acid (5-HPETE), substantially reversed nodulation inhibition caused by dexamethasone and the LOX inhibitors, caffeic acid and esculetin. The COX product, prostaglandin H(2) (PGH(2)), did not reverse the nodulation inhibition by dexamethasone or the COX inhibitor, ibuprofen. None of the inhibitors tested had a significant effect on the phagocytosis of B. bassiana blastospores in vitro. Hemocyte phenoloxidase activity was reduced by dexamethasone, esculetin, and the COX inhibitor, indomethacin. The rescue candidates 5-HPETE and PGH(2) did not reverse the inhibition.     

Cognitive effects of variations in pubertal timing: Is puberty a period of brain organization for human sex-typed cognition?

There is considerable interest in the organizational effects of pubertal sex hormones on human sex-related characteristics. Recent evidence from rodents suggests that there is a decreasing window of sensitivity to sex hormones throughout adolescence. If adolescence also represents a period of brain organization in human beings, then the timing of exposure to sex-typical hormones at puberty should have long-term effects on sex-typed characteristics: individuals with early timing should be more sex-typed than individuals with late timing. We tested this hypothesis in 320 young adults by relating their pubertal timing (retrospective comparison to peers) to cognitive abilities that show sex differences. Results provide partial support for the hypothesis. For men, pubertal timing was inversely related to scores on a test of three-dimensional mental rotations. Effects do not appear to be due to duration of hormone exposure (time since puberty), but other potential influences need further study.     

Using Variable Rate Models to Identify Genes Under Selection in Sequence Pairs: Their Validity and Limitations for EST Sequences

Using likelihood-based variable selection models, we determined if positive selection was acting on 523 EST sequence pairs from two lineages of sunflower and lettuce. Variable rate models are generally not used for comparisons of sequence pairs due to the limited information and the inaccuracy of estimates of specific substitution rates. However, previous studies have shown that the likelihood ratio test (LRT) is reliable for detecting positive selection, even with low numbers of sequences. These analyses identified 56 genes that show a signature of selection, of which 75% were not identified by simpler models that average selection across codons. Subsequent mapping studies in sunflower show four of five of the positively selected genes identified by these methods mapped to domestication QTLs. We discuss the validity and limitations of using variable rate models for comparisons of sequence pairs, as well as the limitations of using ESTs for identification of positively selected genes. [Reviewing Editor: Dr. Rasmus Nielsen]     

Variation in multiple paternity in natural populations of a free-spawning marine invertebrate

For free-spawning marine invertebrates, fertilization processes control the genetic diversity of offspring. Each egg can potentially be fertilized by a sperm from a different male, and hence genetic diversity within a brood varies with levels of multiple paternity. Yet, few studies have characterized the frequency of multiple paternity in natural spawns. We analysed patterns of multiple paternity in two populations of the colonial ascidian Botryllus schlosseri using microsatellites. Because previous studies have shown that at moderate to high population densities, competition among male-phase B. schlosseri colonies results in the nearest male dominating the paternity of a brood, we specifically tested the effect of population density on patterns of paternity. Paternity was estimated using three multilocus indices: minimum number of fathers, counts of sperm haplotypes, and effective paternity (K(E)). Multiple paternity was evident in more than 92% of the broods analysed, but highly variable, with a few broods displaying unequal contributions of different males. We found no effect of population density on multiple paternity, suggesting that other factors may control paternity levels. Indirect benefits from increasing the genetic diversity of broods are a possible explanation for the high level of multiple paternity in this species.     

Coordination of leaf N,anatomy, photosynthetic capacity,and hydraulics enhances evergreen expansive potential

Key message

Reduced leaf longevity, N-fixation, and enhanced hydraulic capacity combined support greater shifts in seasonal photosynthetic capacity of an expansive understory evergreen woody species relative to co-occurring less expansive evergreen species.

Abstract

Physiological functioning typically declines with increased leaf life span. While an evergreen leaf habit is generally associated with reduced leaf N, physiological capacity, and slower growth, most expansive woody species are evergreens and/or N fixers. An evergreen leaf habit enables year-round activity and less investment in carbon and nutrients, while N-fixation enhances photosynthetic capacity. Our objective was to compare anatomy and physiology of three woody evergreens Ilex opaca Aiton (Aquifoliaceae), Kalmia latifolia L. (Ericaceae), and Myrica cerifera (Myricaceae) of varying leaf longevity, N-fixation capability, and known expansive potential in a deciduous forest understory to determine if seasonal physiological performance integrated these factors. We hypothesized that I. opaca (non-expansive) and K. latifolia (moderately expansive), which have longer leaf longevities, would have reduced physiological performance compared to M. cerifera (expansive), which has shorter leaf longevity, and symbiotically fixes atmospheric N. Stomatal conductance to water vapor, photosynthetic and hydraulic capacities, specific leaf area, and leaf %N decreased with increasing leaf life span; however, trends among species were not consistent seasonally. While hydraulic capacity remained constant throughout the year, photosynthetic capacity did not. During the growing season, M. cerifera displayed photosynthetic capacity similar to deciduous species, yet, during the winter, photosynthetic capacity was similar to the slower-growing evergreens. Reduced leaf life span, enhanced hydraulic capacity, and nitrogen fixation support the seasonal shift in photosynthetic capacity observed in M. cerifera. This “hybrid” strategy enables M. cerifera to maximize productivity during months of optimal conditions, thereby promoting rapid growth and expansion in the understory.