OBSERVATIONS ON LEAF AND BUD FORMATION IN HYDROCHARIS MORSUS-RANAE

Regular sequences of leaf and bud formation occur in several members of the Hydrocharitaceae, including Hydrocharis, in which buds are normally formed in the axil of every second leaf of the phyllotactic spiral. Leaf inception begins by periclinal divisions of the inner cells of the 2-layered tunica. Bud formation, which occurs in the apical meristem itself, immediately following the inception of the subtending leaf primordium, begins by divisions in various planes in the corpus, the 2 tunica layers remaining continuous throughout. The young bud meristem soon gives rise to a lateral bud, before leaf formation begins upon it. Because of these and other features, this species is one of considerable morphogenetic interest. Morphogenesis of the whole plant, and in particular the factors controlling the regular sequence of leaf and bud formation, have been and are being investigated experimentally.     

Ephemeral ecological speciation and the latitudinal biodiversity gradient

The richness of biodiversity in the tropics compared to high‐latitude parts of the world forms one of the most globally conspicuous patterns in biology, and yet few hypotheses aim to explain this phenomenon in terms of explicit microevolutionary mechanisms of speciation and extinction. We link population genetic processes of selection and adaptation to speciation and extinction by way of their interaction with environmental factors to drive global scale macroecological patterns. High‐latitude regions are both cradle and grave with respect to species diversification. In particular, we point to a conceptual equivalence of “environmental harshness” and “hard selection” as eco‐evolutionary drivers of local adaptation and ecological speciation. By describing how ecological speciation likely occurs more readily at high latitudes, with such nascent species especially prone to extinction by fusion, we derive the ephemeral ecological speciation hypothesis as an integrative mechanistic explanation for latitudinal gradients in species turnover and the net accumulation of biodiversity.     

Melanic pigmentation and light preference within and between two Drosophila species

Environmental adaptation and species divergence often involve suites of co‐evolving traits. Pigmentation in insects presents a variable, adaptive, and well‐characterized class of phenotypes for which correlations with multiple other traits have been demonstrated. In Drosophila, the pigmentation genes ebony and tan have pleiotropic effects on flies'' response to light, creating the potential for correlated evolution of pigmentation and vision. Here, we investigate differences in light preference within and between two sister species, Drosophila americana and D. novamexicana, which differ in pigmentation in part because of evolution at ebony and tan and occupy environments that differ in many variables including solar radiation. We hypothesized that lighter pigmentation would be correlated with a greater preference for environmental light and tested this hypothesis using a habitat choice experiment. In a first set of experiments, using males of D. novamexicana line N14 and D. americana line A00, the light‐bodied D. novamexicana was found slightly but significantly more often than D. americana in the light habitat. A second experiment, which included additional lines and females as well as males, failed to find any significant difference between D. novamexicana‐N14 and D. americana‐A00. Additionally, the other dark line of D. americana (A04) was found in the light habitat more often than the light‐bodied D. novamexicana‐N14, in contrast to our predictions. However, the lightest line of D. americana, A01, was found substantially and significantly more often in the light habitat than the two darker lines of D. americana, thus providing partial support for our hypothesis. Finally, across all four lines, females were found more often in the light habitat than their more darkly pigmented male counterparts. Additional replication is needed to corroborate these findings and evaluate conflicting results, with the consistent effect of sex within and between species providing an especially intriguing avenue for further research.     

The proximate determinants of sex ratio in C. elegans populations

The soil nematode Caenorhabditis elegans is an example of a species in which self-fertilizing hermaphrodites predominate, but functional males continue to persist--allowing outcrossing to persevere at low levels. Hermaphrodites can produce male progeny as a consequence of sex chromosome non-disjunction or via outcrossing with males. Consequently, the genetics of sex determination coupled with the efficiency by which males find, inseminate and obtain fertilizations with hermaphrodites will influence the frequency at which males and outcrossing occurs in such populations. Behavioural and physiological traits with a heritable basis, as well as ecological characters, may influence male reproductive success and therefore sex ratio. Because sex ratio is tied to male reproductive success, sex ratio greatly affects outcrossing rates, patterns of genetic variation, and the ability of natural selection to act within populations. In this paper we explore the determinants of male frequency in C. elegans with a mathematical model and experimental data. We address the role of the genetic machinery of sex determination via sex chromosome non-disjunction on sex ratio and the influence of physiological components of C. elegans' life history that contribute to variation in sex ratio by way of male reproductive success. Finally, we discuss the short-term and long-term factors that are likely to affect sex ratio and breeding system evolution in species like C. elegans.