Molecular clocks and the incompleteness of the fossil record

Molecular clocks can be evaluated by comparing absolute rates of evolution and by performing relative-rate tests. Typically, calculations of absolute rates are based on earliest observed occurrences in the fossil record. Relative-rate tests, on the other hand, merely require an unambiguous outgroup. A major disadvantage of relative-rate tests is their insensitivity to concomitant and equal rate changes in all lineages. Apparent differences in absolute rates, in turn, may be artifacts that are attributable to an incomplete fossil record.Recently developed methods in quantitative biostratigraphy recognize the incompleteness of the fossil record and allow us to place confidence intervals on the endpoints of taxon ranges. These methods are applicable to taxa whose fossil records are of markedly different quality. When we extend these methods and integrate molecular and paleontologic data, we can test the null hypothesis that seemingly disparate rates of molecular evolution are in fact equal under the simplifying assumption that fossils are randomly and independently distributed over their temporal ranges and that fossils can be accurately placed in a phylogenetic context. We can also estimate the range of ticking rates, if any, that are compatible with known fossil data. Ultimately, more accurate rate estimates for widely divergent taxa should allow for more meaningful comparisons of evolutionary rates.DNA hybridization data for monotremes and marsupials suggest a 17-fold difference for 14 different rate calculations with a mean value of approximately 1% divergence per million years. Variation among marsupials is sevenfold. However, when we apply appropriate statistical tests and make additional allowances for fossils of uncertain taxonomic assignment, etc., all 14 rates are compatible with a molecular clock ticking at approximately 0.4% divergence per million years. In addition, this analysis brings relative- and absolute-rate tests into accord.     

Observations of the pineal region of non-eutherian mammals

Summary A survey has been made of the pineal region of the brain of 11 species of marsupials belonging to 5 families and a species from both families of monotremes.The results show that the pineal body of non-eutherian mammals, although well-defined in all species, has a very varied morphology. Three types of pineal recess occur: (i) a pineal recess in sensu stricto, (ii) an intercommissural pineal recess, and (iii) an infrapineal recess. The existence of nerve fibres which pass through the pineal body and form a spatial link between the habenular and posterior commissures, has been demonstrated in marsupials and monotremes. It is also likely that these animals as well as eutherian mammals possess a nervus conarii. Nerve cells are not a constant feature of the non-eutherian pineal body.The subcommissural organ (SCO) is present in all species. It does not exhibit the same degree of morphological variation as the pineal body. Horizontal sections available for 4 species within 3 families of marsupials show it to be composed of a median portion joined to bilateral protuberances. Large nerve cells occur within the SCO in all marsupial species; they are absent from the monotreme SCO. Tentatively, the relationship of these neurons to the SCO is considered to be merely one of association.The importance of an extended comparative study of this region in non- eutherian mammals in order to add insight into its phylogeny and function is emphasized.     

A comparative proteomic analysis of skin secretions of the tammar wallaby (Macropus eugenii) and the wombat (Vombatus ursinus)

The secretome of the pouch skin of the model marsupial the tammar wallaby, Macropus eugenii has been investigated using techniques of two-dimensional gel electrophoresis, in-gel trypsin digestion followed by nanoliquid chromatography coupled tandem mass spectrometry (LC-MS/MS). Differences in the patterns of secreted proteins were observed in the female pouch at three stages of maturity — reproductively immature; reproductively mature and active and mature, postreproductively active. Skin from the underarm area of mature females had a markedly different secreted protein profile. The greatest diversity of proteins was seen in the mature reproductive pouch and from an opportunistic sample collected from the pouch another mature female marsupial, the common wombat, Vombatus ursinus. A total of 20 proteins were confidently identified from the pouch skin secretions of the tammar wallaby and wombats, whilst 20 proteins were tentatively identified. In all skin secretomes, globins were the most abundant proteins whilst the antimicrobial, dermcidin was detected in the wombat sample. Some proteins such as keratin and actin could be sourced to sloughed and degraded skin cells. A number of proteins were present at such low concentrations that confident identification was not possible. This was compounded by the lack of a comprehensive database of marsupial proteins which constrains the reliability of automated identification protocols.     

Gestational length in the koala, Phascolarctos cinereus

We report a possible case of extended gestation in the koala, Phascolarctos cinereus. Birth of a pouch young was first observed 127 days after the removal of the male from a multi-female colony at Taronga Zoo. No other males were present at that time or had access to the facility. Head measurements and other growth data collected at the time of detection and over the period of pouch life indicates the time from removal of the male and the date of birth to be between 50 and 77 days. DNA fingerprinting using microsatellite loci unambiguously assigned paternity of the pouch young to this male.

These observations suggest either an extended period of gestation of at least 50 days, or activation of a dormant blastocyst from the previous breeding season, as the female entered the period of seasonal oestrus.     

Interactions between aroids and arboreal mammals in the Brazilian Atlantic rainforest

The Brazilian Atlantic rainforest harbours at least 129 non-volant mammal species. There is also a diverse and numerous epiphytic flora associated with this Neotropical rainforest, including several species of Araceae. Nevertheless, knowledge of the ecology of this group of plants and its interaction with animals is poor. In this study we investigated the interactions between epiphytic aroids and arboreal mammals in the Atlantic forest. We carried out the field work between November 1995 and June 1997 in the Parque Estadual Intervales, southeastern Brazil, a still well preserved forest area of approximately 490 km². We analysed faecal samples from primates (Cebus apella and Brachyteles arachnoides), collected during observations in the field, and marsupials (Didelphis aurita, Micoureus demerarae, Gracilinanus microtarsus), collected from animals captured monthly or bi-monthly in live-traps. We recorded 17 species of Araceae in the study area. The seeds of eight species occurred in the faecal samples: Anthurium harrissi, Monstera adansonii, Philodendron corcovadense, P. appendiculatum, P. exymium, P. crassinervium, P. obliquifolium, and Heteropsis oblongifolia. Aroids were present in faecal samples of the primates during most of the year, with a mean monthly percentage of occurrence of about 56.2%. For marsupials, aroids also were a seasonally important food source, as between November and June the mean monthly percentage of occurrence in faecal samples was about 26.5%. Our data indicate that aroids are an important food source for arboreal and semi-arboreal mammals and that these animals may play an important role as seed dispersers of this group. Aroids and arboreal mammals appear to interact more strongly in Atlantic forest than in other Neotropical forests.     

Sexual differentiation in three unconventional mammals: spotted hyenas, elephants and tammar wallabies

The present review explores sexual differentiation in three non-conventional species: the spotted hyena, the elephant and the tammar wallaby, selected because of the natural challenges they present for contemporary understanding of sexual differentiation. According to the prevailing view of mammalian sexual differentiation, originally proposed by Alfred Jost, secretion of androgen and anti-Mullerian hormone (AMH) by the fetal testes during critical stages of development accounts for the full range of sexually dimorphic urogenital traits observed at birth. Jost's concept was subsequently expanded to encompass sexual differentiation of the brain and behavior. Although the central focus of this review involves urogenital development, we assume that the novel mechanisms described in this article have potentially significant implications for sexual differentiation of brain and behavior, a transposition with precedent in the history of this field. Contrary to the "specific" requirements of Jost's formulation, female spotted hyenas and elephants initially develop male-type external genitalia prior to gonadal differentiation. In addition, the administration of anti-androgens to pregnant female spotted hyenas does not prevent the formation of a scrotum, pseudoscrotum, penis or penile clitoris in the offspring of treated females, although it is not yet clear whether the creation of masculine genitalia involves other steroids or whether there is a genetic mechanism bypassing a hormonal mediator. Wallabies, where sexual differentiation occurs in the pouch after birth, provide the most conclusive evidence for direct genetic control of sexual dimorphism, with the scrotum developing only in males and the pouch and mammary glands only in females, before differentiation of the gonads. The development of the pouch and mammary gland in females and the scrotum in males is controlled by genes on the X chromosome. In keeping with the "expanded" version of Jost's formulation, secretion of androgens by the fetal testes provides the best current account of a broad array of sex differences in reproductive morphology and endocrinology of the spotted hyena, and androgens are essential for development of the prostate and penis of the wallaby. But the essential circulating androgen in the male wallaby is 5alpha androstanediol, locally converted in target tissues to DHT, while in the pregnant female hyena, androstenedione, secreted by the maternal ovary, is converted by the placenta to testosterone (and estradiol) and transferred to the developing fetus. Testicular testosterone certainly seems to be responsible for the behavioral phenomenon of musth in male elephants. Both spotted hyenas and elephants display matrilineal social organization, and, in both species, female genital morphology requires feminine cooperation for successful copulation. We conclude that not all aspects of sexual differentiation have been delegated to testicular hormones in these mammals. In addition, we suggest that research on urogenital development in these non-traditional species directs attention to processes that may well be operating during the sexual differentiation of morphology and behavior in more common laboratory mammals, albeit in less dramatic fashion.     

How to build a mammalian super-predator

Questions surrounding the biology of large fossil predators that differ markedly from living forms have long intrigued palaeobiologists. Among such taxa few have excited more interest than sabertooth cats, whose distinctive hypertrophied canines are suggestive of killing behaviors and feeding ecologies that may have departed widely from those of extant carnivores. Moreover, considerable variation among sabertooth species is further suggestive of intriguing differences within the group. Behavior and ecology in another large, extinct mammalian carnivore, the Australian marsupial lion (Thylacoleo carnifex), has also proven contentious. In this study, we assemble a wide range of cranio-dental and postcranial indices in a dataset including machairodont sabertooths, T. carnifex and an extensive sample of extant taxa in order to examine the palaeobiology of these charismatic fossil carnivores. Results of multivariate analyses point to significant relationships between behavior and overall body proportions in extant mammalian carnivores. Postcranial morphologies of two American dirk-tooth species of sabertooth (Smilodon) depart greatly from those of living felids and group most closely with bears among living placentals. Scimitar-tooth species of Homotherium and Machairodus cluster with modern pantherine cats. The marsupial lion groups with Smilodon. If these latter two phylogenetically disparate clades do represent a specialized, robust ecomorph adapted to predation on large prey, then it is a body plan that might be effectively identified on the basis of a handful of ‘bear-like’ postcranial features in combination with a more typically ‘felid-like’ carnassialization of the cheektooth row.     

Molecular Relationships of the New Guinean Bandicoot Genera Microperoryctes and Echymipera (Marsupialia: Peramelina)

The complete 12S rRNA gene was sequenced for multiple exemplars of the New Guinean bandicoot genera Microperoryctes and Echymipera representing many of the currently recognized subspecies. These two genera are resolved as monophyletic sister taxa but there was no genetic support for the family Peroryctidae proposed by Groves and Flannery (1990). Within Microperoryctes, M. papuensis was sister to M. longicauda. Although there was only weak support for recognition of the subspecies M. l. magnus, our results demonstrate the need for further genetic and morphological studies of variation among populations of Microperoryctes. Haplotype relationships within Echymipera did not reflect current species boundaries. Although E. clara was the most divergent species, no clear separation could be made between E. rufescens and E. kalubu (E. kalubu samples from New Britain were consistently more closely allied with the E. rufescens exemplars collected from north and east of the Central Cordillera than with their congeners). We suggest the need for a major morphological reassessment of New Guinean bandicoot relationships.     

Post-weaning cranial ontogeny in two bandicoots (Mammalia,Peramelomorphia, Peramelidae) and comparison with carnivorous marsupials

The ontogeny of the skull has been studied in several marsupial groups such as didelphids, microbiotheriids, and dasyurids. Here, we describe and compare the post-weaning ontogeny of the skull in two species of bandicoots, Echymipera kalubu (Echymiperinae) and Isoodon macrourus (Peramelinae), analyzing specific allometric trends in both groups, describing common (and specific) patterns, and discussing them on functional and phylogenetic grounds. Growth patterns were analyzed both qualitatively and quantitatively, including bivariate and multivariate analyses of allometry. We also evaluated character transformation and phylogenetic signals of the allometric patterns in several groups of marsupials and some placentals. We identified morphological changes between juvenile and adult stages in both species of peramelids, many related to the development of the trophic apparatus. Notable differences were detected in the patterns of growth, suggesting divergences in ontogenetic trajectories between both species. Both bivariate and multivariate methods indicate that positive allometries in E. kalubu apply to longitudinal dimensions, whereas in I. macrourus, positive allometries are restricted to vertical dimensions of the skull. The comparison of the allometric trends of two bandicoots with previously studied taxa reveals that although peramelids exhibit a particularly short gestation period and divergent morphology compared to other marsupials, their pattern does not show any particular trend. Some allometric trends seem to be highly conserved among the species studied, showing weak phylogenetic signal. Marsupials in general do not show particular patterns of post-weaning skull growth compared with placentals.     

DNA/DNA hybridization studies of the carnivorous marsupials. I: The intergeneric relationships of bandicoots (Marsupialia: Perameloidea)

Summary A complete suite of comparisons among six bandicoot species and one outgroup marsupial was generated using the hydroxyapatite chromatography method of DNA/DNA hybridization; heterologous comparisons were also made with three other bandicoot taxa. Matrices of T_m's, modes, and T₅₀Hs were generated and corrected for nonreciprocity, homoplasy, and, in the case of T_m's, normalized percent hybridization; these matrices were analyzed using the FITCH algorithm in Felsenstein's PHYLIP (version 3.1). Uncorrected and nonreciprocity-corrected matrices were also jackknifed and analyzed with FITCH to test for consistency. Finally, sample scores for T_m, mode, and T₅₀H matrices were bootstrapped and then subjected to phylogenetic analysis. These manipulations were carried out, in part, to address criticisms of the statistics used to summarize DNA/DNA hybridization (especially T₅₀H) and the method itself. However, with the exception of an unresolved trichotomy among the twoEchymipera species andPeroryctes longicauda, all trees showed the same branchpoints. Except in the case of the tree generated from reciprocal-corrected T_m data, nodes were stable under jackknifing; and, again excepting the above-mentioned trichotomy, all nodes were supported by 95% or more of the bootstrapped trees. These results suggest that, despite arguments to the contrary, all three summary statistics can be valid for DNA/DNA hybridization data. Of taxonomic interest is the placement ofEchymipera spp. andPeroryctes longicauda together and separate from the more distantPeroryctes raffrayanus; the genusPeroryctes is thus at least paraphyletic. The trees further groupedEchymipera-plus-Peroryctes as the sister group ofIsoodon-plus-Perameles. Limited hybridizations withMacrotis lagotis suggest that its current position as representative of an entirely distinct family of perameloids is correct.This article was presented at the C.S.E.O.L. Conference on DNA-DNA Hybridization and Evolution, Lake Arrowhead, California, May 11–14, 1989