Worldwide diversity in mammalian life histories: Environmental realms and evolutionary adaptations.

Mammalian life history strategies can be characterised by a few axes of variation, conforming a space where species are positioned based on the life history strategies favoured in the environment they exploit. Yet, we still lack global descriptions of the diversity of realised mammalian life history and how this diversity is shaped by the environment. We used six life history traits to build a life history space covering worldwide mammalian adaptation, and we explored how environmental realms (land, air, water) influence mammalian life history strategies. We demonstrate that realms are tightly linked to distinct life history strategies. Aquatic and aerial species predominantly adhere to slower life history strategies, while terrestrial species exhibit faster life histories. Highly encephalised terrestrial species are a notable exception to these patterns. Furthermore, we show that different mode of life may play a significant role in expanding the set of strategies exploitable in the terrestrial realm. Additionally, species transitioning between terrestrial and aquatic realms, such as seals, exhibit intermediate life history strategies. Our results provide compelling evidence of the link between environmental realms and the life history diversity of mammals, highlighting the importance of differences in mode of life to expand life history diversity.

Expression of ribosomal protein genes and regulation of ribosome biosynthesis in Xenopus development.

Studies on ribosome biosynthesis in developing Xenopus oocytes and embryos, and after microinjection of cloned ribosomal-protein genes, have revealed that the synthesis of ribosomal proteins (r-proteins) is controlled by two types of regulation: (1) a post-transcriptional regulation, operated by feedback of the r-proteins themselves, controls processing and stability of r-protein transcripts and thus the amount of the corresponding mRNA present in the cell; and (2) a translational regulation controls the efficiency of utilization of r-protein mRNA (rp-mRNA) in response to the cellular needs for new ribosomes.

XrpFI, an amphibian transcription factor composed of multiple polypeptides immunologically related to the GA-binding protein alpha and beta subunits, is differentially expressed during Xenopus laevis development.

XrpFI, first identified in the extract of Xenopus laevis oocyte nuclei, binds to a proximal sequence of the L14 ribosomal protein gene promoter. Its target sequence, 5'-TAACCGGAAGTTTGT-3', is required to fully activate the promoter, and the two G's of the central motif are essential for factor binding and transcriptional activation; our data also suggest that XrpFI may play a role in cap site positioning. The binding site of XrpFI is homologous to the sequence recognized by the family of ets genes. Antibodies specific for Ets-1 and Ets-2 proteins did not react with XrpFI, but those raised against the rat alpha and beta GA-binding proteins both supershifted the retarded bands formed by XrpFI. The Xenopus polypeptides related to GA-binding protein alpha interact with DNA both as monomers and as heterodimers associated with beta-related proteins. Oocyte nuclei contain multiple forms of alpha- and beta-related proteins: the alpha-like proteins remain throughout development, while the pattern of the beta species changes in the embryonic stages examined. beta-like proteins are undetectable in the cleavage period up to the neurula stage, but at later stages, when ribosomal protein genes are actively transcribed, two beta-related polypeptides reappear.

Clustered and interspersed repetitive DNA sequences in four amphibian species with different genome size.

We have compared the amount of clustered and interspersed repetitive sequences in the genome of four Amphibia with different DNA contents per haploid nucleus: two Anura (Xenopus laevis, 3 pg and Bufo bufo, 7 pg) and two Urodela (Triturus cristatus, 23 pg and Necturus maculosus, 52 pg). High molecular weight DNA of the four species was denatured and reassociated to the same Cot in order to obtain duplex sequences with a similar reiteration frequency. Single-stranded DNA was digested off with the Aspergillus S1 nuclease. DNA was then fractionated according to the molecular weight through an agarose A-50 column. We found that the amount of long repetitive sequences is roughly proportional to the genome size in the four species, while the number of short (about 300 base pairs) repetitive sequences is increased many-fold in the species with the larger DNA content, both in Anura and in Urodela.

Isolation of a DNA fraction highly enriched in transfer RNA genes from Xenopus laevis.

A DNA fraction highly enriched in tRNA genes can be isolated from the Xenopus laevis genome by the use of Ag+/Cs2SO4 density gradients. Ag+ shows a low affinity for some tRNA cistrons, allowing their separation from bulk DNA upon equilibrium centrifugation in a Cs2SO4 density gradient. Contaminating DNA in the resulting tDNA fraction is further removed by two additional CsCl density gradient centrifugations. The final DNA fraction is 60-fold enriched in tRNA genes, compared to the starting DNA material.

Splicing of Xenopus laevis ribosomal protein RNAs is inhibited in vivo by antisera to ribonucleoproteins containing U1 small nuclear RNA.

The activity of antisera against ribonucleoproteins containing U1 small nuclear RNA (Sm and RNP) has been analysed on pol II transcripts in an in vivo system. Xenopus laevis ribosomal protein gene transcripts are accumulated in the form of precursor RNA when either of the two kinds of antisera are injected into the germinal vesicles of X. laevis oocytes before the injection of purified L1 and L14 ribosomal protein genes. No effect on the accumulation of mature histone mRNA is detected when X. laevis histone genes are injected together with the RNP antiserum. These results strongly suggest that U1-RNP complexes play an essential role in intron removal in vivo.

Expression of two Xenopus laevis ribosomal protein genes in injected frog oocytes. A specific splicing block interferes with the L1 RNA maturation.

The expression of two Xenopus laevis ribosomal protein genes (L1 and L14) has been analysed by microinjection of the cloned genomic sequences into frog oocyte nuclei. While the injection of the L14 gene causes the accumulation of the corresponding protein in large excess with respect to that synthesized endogenously, the L1 gene does not. Analysis of the RNA shows that both genes are actively transcribed. The seven-intron-containing L14 transcript is completely processed to a mature form, while two out of nine intron sequences persist in the L1 transcript. This precursor RNA is confined to the nucleus; its accumulation is due to a specific block of splicing operating at the level of two defined introns and not to saturation of the processing apparatus of the oocyte. The different behaviour of the two genes may reflect different mechanisms of regulation which, in the case of the L1 gene, could operate at the level of splicing.

Platelet-derived growth factor-BB stimulates hypertrophy of peritubular smooth muscle cells from rat testis in primary cultures.

The tunica propria of seminiferous tubules contains a particular type of smooth muscle cell (myoid cells) arranged in a contractile epithelioid layer that is responsible for sperm and tubular fluid flow. Unlike other types of smooth muscle (SM) cells, highly purified populations of peritubular smooth muscle cells (PSMC) survive and maintain their contractile phenotype in primary cultures in controlled conditions. We used this culture model to investigate the response of the SM contractile phenotype to prolonged exposure to platelet-derived growth factor (PDGF), one of the main factors involved in vascular SM pathologies. We observed that 4-day continuous exposure of PSMC to PDGF-BB at nanomolar concentrations in plain medium enhances contractile phenotype traits and induces cell hypertrophy without inducing proliferation. In Northern and Western blotting experiments, SM-alpha-actin transcript and protein were found to be markedly increased in the PDGF-BB-treated samples, which is in line with the formation of conspicuous SM-alpha-actin-containing stress fibers. Moreover, binding sites for endothelin-1 were increased, and the calcium response to the contractile agonist, determined in single fura-2-loaded cells, was enhanced. In response to PDGF-BB, the cells underwent immediate, transient contraction, as seen in a scanning electron microscope, followed by a gradual increase in size, as evaluated by cytofluorometry, and enhancement of protein synthesis. The observed pattern of response to PDGF-BB was not accompanied by cell proliferation, as assessed by [3H]thymidine incorporation and direct cell counts. Unlike other SM cell types, in which proliferation and loss of contractile traits are induced by PDGF, chronic treatment of PSMC with this growth factor results in hypertrophy rather than hyperplasia.

Nucleotide sequences of cloned cDNA fragments specific for six Xenopus laevis ribosomal proteins.

We have previously constructed and selected six recombinant plasmids containing cDNA sequences specific for different ribosomal proteins of Xenopus laevis (Bozzoni et al., 1981). DNA cloned in these plasmids have been isolated and sequenced. Amino acid sequences of the corresponding portions of the proteins have been derived from DNA sequences; they are arginine- and lysine-rich as expected for ribosomal proteins. One of the cDNA sequences has an open reading frame also on the strand complementary to the one coding for the ribosomal protein; this fragment has inverted repeats twenty nucleotides lone at the two ends. The codon usage for the six sequences appears to be non-random with some differences among the ribosomal proteins analysed.

Messenger RNA for ribosomal proteins in Xenopus laevis oocytes.

Xenopus laevis oocytes (stage II--III) incorporate [35S]methionine into ribosomal proteins identifiable by two-dimensional electrophoresis. Poly(A)-rich RNA prepared form this source has been translated in a wheat germ cell-free system. Among the products synthesized in vitro 30 are clearly identifiable as ribosomal proteins. Some are not individually resolved, whereas a few of them may lack methionine or may not be correctly processed after the synthesis in vitro. An enrichment of our mRNA preparation for ribosomal protein activity can be obtained by subsequent passages on oligo(dT)-cellulose columns and by selection of a 10--16-S fraction by sucrose gradient sedimentation. We estimate that messenger RNA for ribosomal proteins represents 10--20% of this RNA preparation and can now be utilized for molecular cloning procedures.

Biosynthesis of U16 snoRNA in early development of X. laevis.

The U16 and U18 snoRNAs are encoded in introns of the X.laevis L1 ribosomal protein gene and originate from processing of the pre-mRNA. These snoRNAs are newly synthesized around gastrula stage and progressively accumulate during embryogenesis. We show that the basic factors participating in U16 biosynthesis, such as the endonuclease involved in the cleavage reaction and the factors necessary for stabilization of mature snoRNA are present from very early stages. The use of anucleolate mutants has indicated that the synthesis and accumulation of U16 and U18 snoRNAs is not affected in the absence of ongoing rRNA transcription.

HrpF, a human sequence-specific DNA-binding protein homologous to XrpFI, a Xenopus laevis oocyte transcription factor.

The identification in HeLa nuclei of a novel DNA-binding protein, designated HrpF, is presented. This factor recognizes and binds a sequence of the Xenopus laevis L14 ribosomal protein (r-p) gene promoter bound by the Xenopus r-p transcription factor I (XrpFI). We show here that XrpFI and HrpF share a conserved DNA-binding domain. We also present evidences suggesting that the two factors perform similar functions in the cell. We discuss the hypothesis that closely related factors might be involved in the control of rp-gene transcription in vertebrates.

Xenopus laevis ribosomal protein genes: isolation of recombinant cDNA clones and study of the genomic organization.

Poly-A+ mRNA from Xenopus laevis oocytes, partially enriched for r-protein coding capacity has been used as starting material for preparing a cDNA bank in plasmid pBR322. The clones containing sequences specific for r-proteins have been selected by translation of the complementary mRNAs. Clones for six different r-proteins have been identified and utilized as probes for studying their genomic organization. Two gene copies per haploid genome were found for r-proteins L1, L14, S19, and four-five for protein S1, S8 and L32. Moreover a population polymorphism has been observed for the genomic regions containing sequences for r-protein S1, S8 and L14.

Ribosomal protein production in normal and anucleolate Xenopus embryos: regulation at the posttranscriptional and translational levels.

We have studied the regulation of ribosomal protein (r-protein) synthesis in Xenopus anucleolate mutants, which lack the genes for rRNA. The accumulation of mRNA for the two r-proteins analyzed parallels the controls up to stage 30. This mRNA is mobilized onto polysomes and is translated as in normal embryos, but r-proteins are unstable in the absence of rRNA to assemble with. A translational control of rp-mRNA distribution between polysomes and mRNPs is observed, but this is not due to an autogenous regulation by r-proteins. After stage 30 the amount of rp-mRNA declines specifically in the mutants because the transcripts are unstable. Considering the temporal correlation between this event and the onset of r-protein synthesis we suggest that an autogenous control operates at the level of transcript stability.