Evolutionary origin of the medaka Y chromosome

Genetic sex determination in an XX-XY chromosome system can be realized through a locus on the Y chromosome that makes the undifferentiated gonad develop into a testis. Although this mechanism is widespread, only in two cases so far have the corresponding master male sex-determining genes been identified. One is Sry, which initiates testes determination in most mammals. The other is dmrt1bY (syn. dmy), from the fish medaka, Oryzias latipes. The mammalian Y is roughly estimated to be over 200 million years old. The medaka Y may be considerably younger. A comparative analysis of the genus Oryzias revealed that one sister species of the medaka has dmrt1bY on a homologous Y chromosome, whereas in another closely related species only a non-sex-linked pseudogene is present. In all other species, dmrt1bY was not detected. The divergence time for the different species was determined with mitochondrial DNA sequences. The timing was confirmed by independent calculations based on dmrt1 sequences. We show that the medaka sex-determining gene originated approximately 10 million years ago. This makes dmrt1bY and the corresponding Y chromosome the youngest male sex-determining system, at least in vertebrates, known so far.     

The minimal mammalian Y chromosome - the marsupial Y as a model system

The mammalian X and Y chromosomes are very different in size and gene content. The Y chromosome is much smaller than the X and consists largely of highly repeated non-coding DNA, containing few active genes. The 65-Mb human Y is homologous to the X over two small pseudoautosomal regions which together contain 13 active genes. The heterochromatic distal half of the human Yq is entirely composed of highly repeated non-coding DNA, and even the euchromatic portion of the differential region is largely composed of non-coding repeated sequences, amongst which about 30 active genes are located. The basic marsupial Y chromosome (about 10 Mb) is much smaller than that of humans or other eutherian mammals. It appears to include no PAR, since it does not undergo homologous pairing, synaptonemal complex formation or recombination with the X. We show here that the tiny dunnart Y chromosome does not share cytogenetically detectable sequences with any other chromosome, suggesting that it contains many fewer repetitive DNA sequences than the human or mouse Y chromosomes. However, it shares several genes with the human and/or mouse Y chromosome, including the sex determining gene SRY and the candidate spermatogenesis gene RBMY, implying that the marsupial and eutherian Y are monophyletic. This minimal mammalian Y chromosome might provide a good model Y in which to hunt for new mammalian Y specific genes.     

Zebrafish genes for neuropeptide Y and peptide YY reveal origin by chromosome duplication from an ancestral gene linked to the homeobox cluster

Neuropeptide Y (NPY) and peptide YY (PYY) are related 36-amino acid peptides. NPY is widely distributed in the nervous system and has several physiological roles. PYY serves as an intestinal hormone as well as a neuropeptide. We report here cloning of the npy and pyy genes in zebrafish (Danio rerio). NPY differs at only one to four amino acid positions from NPY in other jawed vertebrates. Zebrafish PYY differs at three positions from PYY from other fishes and at 10 positions from mammals. In situ hybridization showed that neurons containing NPY mRNA have a widespread distribution in the brain, particularly in the telencephalon, optic tectum, and rhombencephalon. PYY mRNA was found mainly in brainstem neurons, as reported previously for vertebrates as divergent as the rat and the lamprey, suggesting an essential role for PYY in these neurons. PYY mRNA was observed also in the telencephalon. These results were confirmed by immunocytochemistry. As in the human, the npy gene is located adjacent to homeobox (hox) gene cluster A (copy a in zebrafish), whereas the pyy gene is located close to hoxBa. This suggests that npy and pyy arose from a common ancestral gene in a chromosomal duplication event that also involved the hox gene clusters. As zebrafish has seven hox clusters, it is possible that additional NPY family genes exist or have existed. Also, the NPY receptor system seems to be more complex in zebrafish than in mammals, with at least two receptor genes without known mammalian orthologues.     

The Y chromosome that lost the male-determining function behaves as an X chromosome in the medaka fish, Oryzias latipes

The medaka, Oryzias latipes, has an XX/XY sex-determination system, and a Y-linked DM-domain gene, DMY, is the sex-determining gene in this species. Since DMY appears to have arisen from a duplicated copy of the autosomal DMRT1 gene approximately 10 million years ago, the medaka Y chromosome is considered to be one of the youngest male-determining chromosomes in vertebrates. In the screening process of sex-reversal mutants from wild populations, we found a population that contained a number of XY females. PCR, direct sequencing, and RT-PCR analyses revealed two different null DMY mutations in this population. One mutation caused loss of expression during the sex-determining period, while the other comprised a large deletion in putative functional domains. YY females with the mutant-type DMY genes on their Y chromosomes were fully fertile, indicating that the X and Y chromosomes were functionally the same except for the male-determining function. In addition, we investigated the frequencies of the sex chromosome types in this population over four successive generations. The Y chromosomes bearing the mutant-type DMY genes were detected every year with no significant differences in their frequencies. These results demonstrate that aberrant Y chromosomes behaving as X chromosomes have been maintained in this population.     

The origin of Mosuo people as revealed by mtDNA and Y chromosome variation

The Mosuo, living in the Lugu Lake area in northwest Yunnan Province, China, is the only matriarchal population in China. The Mosuo was officially identified as Naxi nationality although its relationship with Naxi remains controversial. We studied the genetic relationship between the Mosuo and five other ethnic groups currently residing in northwest Yunnan, i.e. Naxi, Tibetan, Bai, Yi and Pumi, by typing the genetic variations in mtDNA HVS1 and 21 Y chromosome markers (13 SNPs & 8 STR markers). We showed that the maternal lineages of the Mosuo bear the strongest resemblance with those found in Naxi while its paternal lineages are more similar to those that are prevalent in Yunnan Tibetan. The marked difference between paternal and maternal lineages may be attributable to the genetic history, matriarchal structure, and visiting marriage.     

Evolutionary origin and diversification of the mammalian CD1 antigen genes.

CD1 antigens are cell-surface glycoproteins which have a molecular structure which is similar (consisting of extracellular domains alpha 1, alpha 2, and alpha 3, a transmembrane portion, and a cytoplasmic tail) to that of class I MHC molecules. Phylogenetic analysis of mammalian CD1 DNA sequences revealed that these genes are more closely related to the class I major histocompatibility complex (MHC) than to the class II MHC and that mammalian genes are more closely related to avian class I MHC genes than they are to mammalian class I MHC genes. The CD1 genes form a multigene family with different numbers of genes in different species (five in human, eight in rabbit, and two in mouse). Known CD1 genes are grouped into the following three families, on the basis of evolutionary relationship: (1) the human HCD1B gene and a partial sequence from the domestic rabbit, (2) the human HCD1A and HCD1C genes, and (3) the human HCD1D and HCD1E genes plus the two mouse genes and a sequence from the cottontail rabbit. The alpha 1 and alpha 2 domains of CD1 are much less conserved at the amino acid level than are the corresponding domains of class I MHC molecules, but the alpha 3 domain of CD1 seems to be still more conserved than the well-conserved alpha 3 domain of class I MHC molecules. Furthermore, in the human CD1 gene family, interlocus exon exchange has homogenized alpha 3 domains of all CD1 genes except HCD1C.     

The fast–slow continuum and mammalian life-history patterns: an empirical evaluation

The fast–slow continuum hypothesis has been proposed to explain the diversity of life-history patterns exhibited by biological populations, but the quantification and population-dynamic consequences of the continuum has remained unclear. I used the ratio of fertility rate to age at first reproduction (F/α ratio) to quantify the tempo of life-history of 138 populations of mammals, and investigated the life-history and population-dynamic consequences of being “fast” or “slow”. “Fast” mammals (F/α>0.60) were characterized by early maturity, short lifespans, low survival rates, and high fertility and projected population growth rate (λ) compared to “slow” (F/α<0.15) mammals. In “fast” populations, λ was overwhelmingly most sensitive to changes in reproductive parameters (age at first reproduction and fertility rates) and relatively insensitive to changes in survival rates. In “slow” populations, λ was very sensitive to changes in juvenile or adult survival rates, and relatively insensitive to changes in reproductive parameters. The pattern of relationships between the F/α ratio and life-history variables, λ, and elasticity of λ to changes in life-history variables persisted even after the effects of body size and phylogeny were statistically removed. These results suggest that fast–slow continuum in mammalian life-history is independent of body size or phylogeny, that the F/α ratio adequately quantifies the position of a population along a fast–slow continuum, and that the tempo of life- histories has substantial population-dynamic consequences.

Zusammenfassung

Die r-K-Kontinuum-Hypothese wurde aufgestellt, um die Diversität von ,,life-history“-Mustern biologischer Populationen zu erklären, aber die Quantifizierung und die Kosnsequenzen für die Populationsdynamik des Kontinuums blieben unklar. Ich benutze das verhältnis der Fortpflanzungsrate zum Fortpflanzungsalter (F/α-Verhältnis) um die Geschwindigkeit der ,,life-history“ von 138 Populationen von Säugetieren zu quantifizieren und untersuchte die Konsequenzen fur die Lebensweise sowie die Populationsdynamik des,,schnell“oder,,langsam “-Seins. ,,Schnelle“Säugetiere (F/α>0.60) waren durch eine frühe Reife, kurze Lebenszeiten, geringe Überlebensraten sowie durch eine große Fertilität und hochgerechnete Populationswachstumsrate (λ) im Vergleich zu ,,langsamen“(F/α<0.15) Säugetieren charakterisiert. In ,,schnellen“ Population reagierte (λ) überwältigend sensibel auf Änderungen in den Fortpflanzungsparametern (Fortpflanzunsalter und Fertilitätsrate) und relativ gering auf Veräanderungen in der Überlebensrate. In ,,langsamen“ Populationen reagierte (λ) sehr sensibel auf Veräanderungen in den reproduktiven Parametern. Das Muster der Beziehung zwischen dem (F/α-Verhältnis) und den Variablen der ,,life-history“,λ, und die Elastizität von λ gegenüber Veränderungen in den variablen der Lebensweise bliev sogar bestehen, nachdem die Effekte von Körpergröße und Phylogenese statistisch eliminiert wurden. Diese Ergebnisse lassen vermuten, dass das r-K-Kontinuum in der ,,life-history“der Säugetiere unabhängig von der Körpergröße und Phylogenie ist, dass das F/α-Verhältnis die Position einer Population im r-K-Kontinuum quantifiziert und dass die Geschwindigkeit der,,life-history“beachtliche konsequenzen fur die Populationsdynamik hat.     

Targeted ‘knockdown’ of spliceosome function in mammalian cells

The existence of two sophisticated parallel splicing machineries in multicellular organisms has raised intriguing questions—ranging from their impact on proteome expansion to the evolution of splicing and of metazoan genomes. Exploring roles for the distinct splicing systems in vivo has, however, been restricted by the lack of techniques to selectively inhibit their function in cells. In this study, we show that morpholino oligomers complementary to the branch-site recognition elements of U2 or U12 small nuclear RNA specifically suppress the function of the two splicing systems in mammalian cells. The data provide the first evidence for a role of distinct spliceosomes in pre-mRNA splicing from endogenous mammalian genes and establish a tool to define roles for the different splicing machineries in vivo.     

The mushroom bodies – prominent brain centres of arthropods and annelids with enigmatic evolutionary origin

Loesel, R. and Heuer, C.M. 2010. The mushroom bodies – prominent brain centres of arthropods and annelids with enigmatic evolutionary origin. —Acta Zoologica (Stockholm) 91 : 29–34 Mushroom bodies (MBs) are the most prominent and conspicuous neuropils in the brain of arthropods, onychophorans and vagile polychaete annelids but have not been described in any other animal group with complex brain architecture. Due to a number of unique neuroanatomical characters MBs can easily be identified and distinguished from other brain centres. However, their evolutionary origin and the question whether MBs are homologous structures is still under debate. This paper will briefly summarize the available morphological data and their implications with respect to the molecular evidence on early metazoan radiation. Unraveling the origin of MBs is an example of the challenges neurophylogenists will face in the future, especially so since it will signify a major step towards reconstructing early metazoan brain evolution.