Exploring the evolution of environmental sex determination, especially in reptiles

Environmental sex determination has been documented in a variety of organisms for many decades and the adaptive significance of this unusual sex-determining mechanism has been clarified empirically in most cases. In contrast, temperature-dependent sex determination (TSD) in amniote vertebrates, first noted 40 years ago in a lizard, has defied a general satisfactory evolutionary explanation despite considerable research effort. After briefly reviewing relevant theory and prior empirical work, we draw attention to recent comparative analyses that illuminate the evolutionary history of TSD in amniote vertebrates and point to clear avenues for future research on this challenging topic. To that end, we then highlight the latest empirical findings in lizards and turtles, as well as promising experimental results from a model organism, that portend an exciting future of progress in finally elucidating the evolutionary cause(s) and significance of TSD.     

Evolution of oestrogen functions in vertebrates

Steroidal oestrogens have been isolated from marine and terrestrial animals representative of all major classes of vertebrates including fish, amphibians, reptiles, birds and mammals. In general, oestrogens are responsible for most features characteristic of the female sex of a species, such as metabolic, behavioural and morphological changes during the stages of reproduction; they also support several processes in males. The evolution of the hormonal system always involves both the ligand and its sites of interaction. In the case of oestrogens, the steroid producing enzymes, mainly the aromatase complex, and the oestrogen receptor belong together within their co-evolution. The finding of oestrogenic steroids, the more recent identification of aromatase and receptor genes and their expression fit together, thereby confirming the importance for all vertebrates. Within the present paper, the evolution of the physiological functions of oestrogens from oviparous vertebrates to Eutherian mammals, oestrogen biosynthesis, metabolization and signalling pathways will be reviewed in detail.     

Light-dependent K(+) channels in the mollusc Onchidium simple photoreceptors are opened by cGMP

Light-dependent K(+) channels underlying a hyperpolarizing response of one extraocular (simple) photoreceptor, Ip-2 cell, in the marine mollusc Onchidium ganglion were examined using cell-attached and inside-out patch-clamp techniques. A previous report (Gotow, T., T. Nishi, and H. Kijima. 1994. Brain Res. 662:268-272) showed that a depolarizing response of the other simple photoreceptor, A-P-1 cell, results from closing of the light-dependent K(+) channels that are activated by cGMP. In the cell-attached patch recordings of Ip-2 cells, external artificial seawater (ASW) was replaced with a modified ASW containing 150 mM K(+) and 200 mM Mg(2+) to suppress any synaptic input and to maintain the membrane potential constant. When Ip-2 cells were equilibrated with this modified ASW, the internal K(+) concentration was estimated to be 260 mM. Light-dependent single-channels in the cell-attached patch on these cells were opened by light but scarcely by voltage. After confirming the light-dependent channel activity in the cell-attached patches, an application of cGMP to the excised inside-out patches newly activated a channel that disappeared on removal of cGMP. Open and closed time distributions of this cGMP-activated channel could be described by the sum of two exponents with time constants tau(o1), tau(o2) and tau(c1), tau(c2), respectively, similar to those of the light-dependent channel. In both the channels, tau(o1) and tau(o2) in ms ranges were similar to each other, although tau(c2) over tens of millisecond ranges was different. tau(o1), tau(o2), and the mean open time tau(o) were both independent of light intensity, cGMP concentration, and voltage. In both channels, the open probability increased as the membrane was depolarized, without changing any of tau(o2) or tau(o). In both, the reversal potentials using 200- and 450-mM K(+)-filled pipettes were close to the K(+) equilibrium potentials, suggesting that both the channels are primarily K(+) selective. Both the mean values of the channel conductance were estimated to be the same at 62 and 91 pS in 200- and 450-mM K(+) pipettes at nearly 0 mV, respectively. Combining these findings with those in the above former report, it is concluded that cGMP is a second messenger which opens the light-dependent K(+) channel of Ip-2 to cause hyperpolarization, and that the channel is the same as that of A-P-1 closed by light.     

Sexual size dimorphism in parasitic oxyurid nematodes

As in many invertebrates, female oxyurids are larger than male. Sexual size dimorphism (SSD) of oxyurid nematodes (the hosts of which are both invertebrate and vertebrate), is investigated regarding body size of both host and parasite. SSD of parasites appeared to be weakly, but not significandy, correlated with invertebrate and vertebrate host body size. However, this study reveals a different pattern for SSD with respect to either type of host. SSD does not increase in tandem with body size in vertebrate parasites either at the level of species or genus. SSD is much more pronounced in Syphaciidae than in Heteroxynematidae, two families of vertebrate parasites exhibiting different modes of transmission (members of the Syphaciidae are transmitted through perianal contamination). SSD is investigated in one monophyletic group of parasites of primates, for which a phylogeny is known. Independent comparisons method is used and we find that the body size of female parasite is strongly correlated with that of the male. The hypoallometry (slope<1) of the relationship suggests that the SSD is not linked to an increase of parasite body size. Moreover, there is no influence of host body size on parasite SSD. The pattern in parasites of invertebrates is different. First, SSD has been found to increase with parasite body size in two groups of invertebrate parasites: the oxyurids of Dictyoptera and Coleoptera. Second, female body size of invertebrate parasites is not correlated with male body size either at genus or species level. Finally, the evolution of SSD is discussed in relation to the demographic patterns of invertebrate parasites and the haplodiploid mode of reproduction of these parasitic nematodes.     

Taphonomy of the south Brazilian Triassic herpetofauna: fossilization mode and implications for morphological studies

Skulls and limb bones of reptiles from Mid-Triassic rocks in southern Brazil show striking morphological and volumetric differences among specimens from the same taxonomic group; this is caused by early calcite cementation. Petrographic analysis of 40 thin sections of selected fossil bones demonstrates that the main agent of fossilization was precipitation of calcite and minor hematite in the pores of bones during early burial mineralization. The framework of the bones has been broken and replaced by calcite (and hematite), beginning in the spongy inner part of the bone (the cancellous bone) and gradually extending into the compact external layers. Three taphonomic groups of fossil bones are recognized: I – those almost entirely retaining their original structure and shape; II – those whose internal structure is destroyed, but which still have solid outer 'bone surface', and III – those composed of fragments of bone apatite 'floating' in a carbonate matrix. Destruction of the bone structure was caused by the displacive force of calcite crystallization, which typically occurs at shallow burial, during early diagenesis. This process occurs in the vadose zone and requires markedly alternating wet and dry seasons, which indicates a semi-arid paleoclimate for the south Brazilian Mid-Triassic, an inference consistent with other paleoclimatic data. Diagenesis can significantly alter the shape and size of bones and cause considerable morphological variation within the same fossil group, possibly leading to taxonomic misinterpretation.     

燕辽生物群脊椎动物的多样性及与其他生物群的对比分析*

燕辽生物群已发现脊椎动物54属58种, 包括鱼类、两栖类、爬行类、哺乳类等, 但其脊椎动物多样性及其成因机制还未有详细研究。本文对该生物群脊椎动物进行统计分析, 并与同时代的其他生物群脊椎动物类型进行对比, 这为认识燕辽生物群脊椎动物的多样性及其成因提供了重要的证据。早期代表道虎沟生物群与晚期代表玲珑塔生物群虽存在时代上的传承关系, 但生物组合特征明显不同。对比燕辽生物群与相近时代的新疆五彩湾动物群和四川大山铺恐龙动物群, 脊椎动物组合特征差异显著。燕辽生物群恐龙类群主要以小型兽脚类恐龙为主, 还包括一些小型鸟臀类恐龙。另外还具有非常丰富的翼龙和哺乳动物。脊椎动物生态多样性高, 适应飞行、树栖、水生、穴居等多种生活方式, 但是脊椎动物的类型与同时代的相近地区明显不同。翼龙、恐龙和哺乳动物等类群都展现出独特的生物组合特征。有证据表明该时期东亚地区与其他地区可能存在一定程度的地理隔离, 结合陆生脊椎动物组合特征推测燕辽生物群脊椎动物与外界可能存在一定的交流障碍。     

A eutherian mammal from the Early Cretaceous of Russia and biostratigraphy of the Asian Early Cretaceous vertebrate assemblages

Prokennalestes abramovi n.sp. is described based on M2 from the upper Barremian-middle Aptian (Early Cretaceous) Mogoito locality in Transbaikalia, Russia. It differs from the Mongolian species of Prokennalestes (Khoboor, early Albian) by a combination of one primitive character (steeper and shorter lingual slopes of the paracone and metacone), one more derived character (larger size), and some characters of uncertain polarity (small preparastyle, lack of labial cuspules along the ectoflexus). P. abramovi n.sp. is the oldest eutherian mammal so far described, and its discovery extends the known geological range of Eutheria by 10-15 Ma. The Tsagantsabian land-vertebrate biochron can be defined by a dominance of sinemyid turtles; the Khukhtekian by a dominance of macrobaenids.