Temperature-dependent sex determination in reptiles: Proximate mechanisms,ultimate outcomes,and practical applications

In many egg-laying reptiles, the incubation temperature of the egg determines the sex of the offspring, a process known as temperature-dependent sex determination (TSD). In TSD sex determination is an “all or none” process and intersexes are rarely formed. How is the external signal of temperature transduced into a genetic signal that determines gonadal sex and channels sexual development? Studies with the red-eared slider turtle have focused on the physiological, biochemical, and molecular cascades initiated by the temperature signal. Both male and female development are active processes—rather than the crganized/default system characteristic of vertebrates with genotypic sex determination—that require simultaneous activation and suppression of testis- and ovary-determining cascades for normal sex determination. It appears that temperature accomplishes this end by acting on genes encoaing for steroidogenic enzymes and steroid hormone receptors and modifying the endocrine microenvironment in the embryo. The temperature experienced in development also has long-term functional outcomes in addition to sex determination. Research with the leopard gecko indicates that incubation temperature as well as steroid hormones serve as organizers in shaping the adult phenotype, with temperature modulating sex hormone action in sexual differentiation. Finally, practical applications of this research have emerged for the conservation and restoration of endangered egg-laying reptiles as well as the embryonic development of reptiles as biomarkers to monitor the estrogenic effects of common environmental contaminants. © 1994 Wiley-Liss, Inc.     

Pollen wall development in Vigna vexillata I. Characterization of wall layers

Light and electron microscope observations characterized the layers that comprise Vigna vexillata L. pollen walls, and identified the timing of their development. Exine sculpturings form an unusually coarse ektexinous reticulum. The structure of the ektexine is granular; this differs from the columellate/tectate type of structure typical of most angiosperm pollen. The ektexine overlies a homogeneous-to-lamellar, electron-dense endexine, which in turn surrounds a thick, microfibrillar intine. Pollen grains are triporate and operculate, with Zwischenkörper and thickened intine underlying the apertures. The ektexine forms during the tetrad period of microspore development, the endexine and Zwischenkörper during the free microspore stage, and the intine during the bicelled (pollen) stage. Coarsely reticulate exine sculpturings and the granular structure of the patterned exine wall of the pollen grains are features that make this species suitable for detailed studies of pollen wall pattern formation.     

Auto and transactivation of FGF expression: Potential mechanism for regulation of myogenic differentiation

Summary Fibroblast growth factors (FGFs) are potent inhibitors of myogenic differentiation. The recent observation that the endogenous expression of acidic and basic FGF by myogenic cells decreases coordinately with differentiation suggests a regulatory role for these growth factors in myogenesis. Inasmuch as other proteins known to influence myogenesis (e.g., MyoD1 and myogenin) activate their own expression as well as the expression of other members of their family, we hypothesized that the FGFs might be capable of similar autoregulation. We examined the effect of exogenously supplied FGF on the abundance of the mRNAs encoding acidic and basic FGF in Sol 8 myoblasts, and demonstrate that either acidic or basic FGF stimulate, through paracrine mechanisms, the accumulation of the mRNAs encoding both of these FGFs. Thus FGFs can auto- and transregulate their own expression in a manner analogous to that observed for the myogenic determination proteins. In addition, similar to that previously observed for MyoD1, both acidic and basic FGF suppress myogenin expression in myoblasts. These results suggest two mechanisms whereby endogenously produced FGFs participate in the maintenance of the undifferentiated state of myogenic cells. These data provide support for paracrine, and suggest potential autocrine, roles for FGFs in the regulation of myogenic differentiation.     

Role of menaquinone in the reduction of fumarate, nitrate, iron(III) and manganese(IV) by Shewanella putrefaciens MR-1

Abstract Mutants of Shewanella putrefaciens MR-1 deficient in menaquinone and methylmenaquinone, but which have wild-type levels of ubiquinone, retain the ability to use trimethylamine N -oxide as an electron acceptor, but they lose the ability to use nitrate, iron(III), and fumarate as electron acceptors. These mutants also show a reduced rate of manganese(IV) reduction. One of these mutants could be restored to essentially wild-type phenotype by supplementing the medium with 1,4-dihydroxy-2-naphthoic acid. A requirement for naphthoquinones in iron(III) reduction and a preference for naphthoquinones in manganese(IV) reduction provide further support that the metal reducing systems in MR-1 are linked to anaerobic respiration.     

Biosynthesis and function of membrane bound and secreted forms of recombinant CD11b/CD18 (Mac-1)

Full-length (membrane bound) and truncated (secreted) forms of the beta 2 integrin heterodimer, CD11b/CD18 (Mac-1), were expressed in a human kidney cell line (293) that normally does not express leukocyte adhesion molecules (Leu-CAMs). The biosynthesis of recombinant Mac-1 in 293 cells differed from that reported for leukocytes in that heterodimer formation was not required for CD11b to be exported to the cell surface. A stable cell line was constructed that constitutively secreted the recombinant, truncated Mac-1 heterodimer into growth conditioned cell culture medium. A novel monoclonal antibody that enabled an immunoaffinity method for the selective purification of recombinant Mac-1 heterodimers was identified. Sufficient protein was purified to allow the first measurement of the 50% inhibitory concentration (IC₅₀) for CD11b/CD18 and for the direct comparison of the inhibitory activity of recombinant soluble Mac-1 with that of various CD18 and CD11b specific monoclonal antibodies. Purified recombinant soluble Mac-1 inhibited the binding of neutrophils, activated by opsonized zymosan or fMet-Leu-Phe peptide, to human umbilical vein endothelial cells. Similarly, the recombinant integrin was effective in inhibiting the binding of unactivated neutrophils to tumor necrosis factor (TNF-alpha) activated endothelial cells. The availability of an abundant source of purified, biologically active Mac-1 will enable direct physical and chemical investigations into the relationship between the structure and function of this leukocyte adhesion molecule.     

Time-Dependent Effects of Progressive Gamma -Linolenate Feeding on Hyperphagia,Weight Gain,and Erythrocyte Fatty Acid Composition During Growth of Zucker Obese Rats

Obese Zucker rats (fa/fa) have low levels of arachidonic acid (AA) in liver phospholipids (PL). We have previously shown that a 70% gamma-linolenate concentrate (GLA; an AA intermediate) fed at a fixed dose (0.07 g/day) normalized hepatic PL AA and reduced weight gain selectively in the obese animals. In a follow-up study, 16 obese (fa/fa) and 16 lean (Fa/Fa) 4-week-old male rats were randomized into 4 groups of 8 each and gavaged daily with soybean oil (SOY) containing 55% 18:2ω6 (an AA precursor) or GLA, using a progressive dose (≤ 5% of total calories) based on body weight. A defined diet with 11% of energy as SOY was fed ad libitum for 60 days. GLA obese had lower body weight (p<0.0001) and 60-day cumulative food intake (p<0.05) compared to SOY obese, but neither parameter differed between the lean groups. For the last twenty days cumulative food intake was identical for GLA obese and SOY lean, whereas SOY obese consumed 18% more (p<0.05). Thus the progressive dose of GLA selectively suppressed hyperphagia in obese Zucker rats. Erythrocytes collected at 15-day intervals showed parallel increases in AA in both genotypes over time, suggesting normal AA availability during rapid growth. Thus, the reduced PL AA in the livers from the obese rats probably reflects impaired distribution in selected tissues rather than reduced hepatic production. Due to the potential health risks of enriching tissue lipids with AA, great caution is advised in considering GLA as therapy for human obesity.