Patterns and sources of variation in Daphnia phosphorus content in nature

It has recently been shown that Daphnia can vary in the phosphorus (P)-content of their body tissues, but the relative importance of genetic versus environmental causes for this variation is unexplored. We measured variation in P-content (as % body mass) of Daphnia from eight lakes and conducted experiments to contrast three sources of variation: interspecific variation, clonal variation and phenotypic plasticity. Daphnia P-content decreased with increasing seston C:P ratio across lakes. This relationship reflected both inter- and intraspecific variation. Daphnia parvula and D. dubia exhibited high P-content and were found in shallow lakes with low C:P seston, whereas D. pulicaria had low P-content and was found in deep, stratified lakes having high C:P seston. Populations of D. dentifera spanned this lake gradient and exhibited P-content that was negatively related to seston C:P. Evidence for phenotypic plasticity came from experiments with D. pulicaria and D. dentifera collected from a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations of D. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-species clonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in the P-content of individual clones of D. pulex (range 1.54–1.05%) and D. mendotae (1.51–1.07%) over a gradient in dietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggests that variation in Daphnia P-content from laboratory experiments needs to be interpreted with caution. These results also show that phenotypic variation in response to environment can be a larger source of variation in P-content than genetic differences within or among species.     

The acquisition of resistance to ecdysteroids in cultured Drosophila cells

Drosophila melanogaster K_c cells become refractory toward ecdysteroids after 4 days of exposure to the molting hormone, 20-OH-ecdysone. Associated with the appearance of hormonal insensitivity is a loss of ecdysteroid receptors. Hormone-resistant cells maintain a low level of receptor that is indistinguishable from that of responsive, hormonally naive cells. After extended periods in culture, ecdysteroid receptor content in previously exposed cells returns to that of naive control cells. The reappearance of receptor is coincident with the resumption of hormonally induced growth inhibition.     

The two-step model of bacterial UV mutagenesis

Recent results are discussed which have led to a two-step model for UV mutagenesis in excision-deficient Escherichia coli. After exposure to UV, the replication fork is assumed to continue until immediately before certain photoproducts where it stops and leaves a gap which cannot be dealt with by recombination repair. In the first (misincorporation) step, bases (a proportion of which are ‘wrong’) are postulated to be inserted opposite the photoproduct under the direct influence of the recA gene product. These misincorporated bases can be revealed as mutations by delayed photoreversal in umuD, C and lexA (ind⁻) bacteria. Their level is determined by the particular allele of recA that is present (recA441 > recA⁺ > recA430) and their rate of formation by the amount of recA protein in the cell and the degree of enrichment of the medium. No other protein needs to be synthesized for this step to occur. The second (bypass) step requires induced levels of the products of the umuD and C genes which are postulated to facilitate continued DNA synthesis on the priming end opposite the photoproduct. In principle, further errors could be made at this stage which might appear as ‘hitch-hiking’ rather than ‘targeted’ mutations.     

Sexual Orientation Disparities in Weight Status in Adolescence: Findings From a Prospective Study

A growing number of studies among adult women have documented disparities in overweight adversely affecting lesbian and bisexual women, but few studies have examined sexual orientation–related patterns in weight status among men or adolescents. We examined sexual orientation group trends in BMI (kg/m²), BMI Z‐scores, and overweight using 56,990 observations from 13,785 adolescent females and males in the Growing Up Today Study (GUTS), a large prospective cohort of US youth. Participants provided self‐reported information from six waves of questionnaire data collection from 1998 to 2005. Gender‐stratified linear regression models were used to estimate BMI and BMI Z‐scores and modified Poisson regression models to estimate risk ratios for overweight, controlling for age and race/ethnicity, with heterosexuals as the referent group. Among females, we observed fairly consistently elevated BMI in all sexual orientation minority groups relative to heterosexual peers. In contrast, among males we documented a sexual‐orientation‐by‐age interaction indicating steeper increases in BMI with age from early‐to‐late adolescence in heterosexuals relative to sexual orientation minorities. Additional prospective research is needed to understand the determinants of observed sexual orientation disparities and to inform appropriate preventive and treatment interventions. The long‐term health consequences of overweight are well‐documented and over time are likely to exact a high toll on populations with elevated rates.