Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Diversification”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,027 records · Page 57Linked to original sources
PubMed1992

Origins, diversification, and historical structure of the helminth fauna inhabiting neotropical freshwater stingrays (Potamotrygonidae).

Members of the freshwater stingray family Potamotrygonidae occur throughout the major river systems of eastern South America that empty into the Atlantic Ocean. Ichthyologists have tended to assume that the ancestor of the potamotrygonids was an Atlantic marine or euryhaline stingray that dispersed into freshwater, presumably during the last marine ingression 3-5 million years ago. The helminth parasites that inhabit potamotrygonids suggest an alternative perspective on their origin. Phylogenetic and biogeographic analysis of the helminths inhabiting potamotrygonids suggest that the hosts are derived from an ancestral Pacific urolophid stingray that was trapped in freshwater by the uplifting of the Andes beginning perhaps as early as the early Cretaceous period and ending by the mid-Miocene epoch, changing the course of the Amazon River, which previously had flowed into the Pacific Ocean.

Animals↗
PubMed2005

Fine structure of juvenile feathers of the zebrafinch in relation to the evolution and diversification of pennaceous feathers.

The present ultrastructural study describes the formation of feather ramification in developing juvenile feathers of the zebrafinch, a small passeraceous bird. The study stresses the importance of the detailed knowledge on the cell structure of barb ridges for the understanding of feather development and evolution. Feather formation depends on the morphogenesis of long barb ridges, in which cells are displaced into lateral barbule plates and a medial barb cells region. These cells merge into long chains and form a syncitium organized in a ramified structure that preserves the original cell disposition within the barb ridge. Barb vane ridge cells surround barb and barbule cells. Barbules separate after the degeneration of barb vane ridge cells. In barbule cells the formation of hooklets resembles the process of formation of climbing setae of digital pads of some lizards. The cytoplasm of barb vane ridge cells is localized among tile-like overlapped barbule cells that form barbule chains, and maintains a serrated outline. When barb vane ridge cells degenerate among keratinized barbules, keratinized hooklets remain. Hooklets allow the ordered grasping of barbules to form a close and planar vane of feathers. The rachis of juvenile feathers seems to be formed from the fusion of two or more barb ridges localized in the dorsal part of the follicle, but the process of fusion is unclear. Juvenile and adult feathers contain the same type of feather keratin present in downfeathers: this indicates that stem cells for the regeneration of a new feather remain in the follicle after shedding of downfeathers. The presence of embryonic organelles (periderm granules) in barb vane ridge cells of juvenile feathers further indicates that also stem cells for the regeneration of the latter cells remain in the follicle. Molting feathers are therefore derived from stem cells. The permanence of stem cells in the follicle and the modulation of barb ridges dimension and fusion into different patterns allow the production of different feather morphotypes such as contour, filoplumes, semiplumes, and bristles.

Age Factors↗
PubMed1991

Molecular genetics of human erythrocyte MiIII and MiVI glycophorins. Use of a pseudoexon in construction of two delta-alpha-delta hybrid genes resulting in antigenic diversification.

Human glycophorins alpha and delta (or A and B) specify the MNSs blood group antigens; they exhibit considerable structural variation among populations. We show that two variant phenotypes of Miltenberger class III and VI are encoded by similar hybrid glycophorin genes in a delta-alpha-delta arrangement. Restriction mapping identified altered fragments unique to the MiIII and MiVI genes. Genomic sequences spanning exons 2 to 4 of the two genes were obtained by allele-specific polymerase chain reaction. Restriction analysis and direct sequencing of the amplified DNA revealed that MiIII and MiVI genes are identical to the delta gene except that, in both, an internal segment of the delta gene has been replaced by its homologous counterpart of the alpha gene, resulting in a delta-alpha-delta hybrid structure. In the process of hybrid formation a portion of alpha exon 3 and intron 3, that carries a functional 5' splicing signal, has been fused to an exon-like sequence in the delta gene that retains a 3' but lacks a 5' splicing signal. These rearrangements created a composite exon resulting in the expression of the ordinarily unexpressed delta gene sequence and conferred the hybrid proteins with new antigenic specificities. The expression of this sequence in MiIII glycophorin is directly demonstrated by protein sequencing. MiIII and MiVI genes differ in the location of upstream (delta-alpha) and downstream (alpha-delta) breakpoints and in the length of sequence replacement. The delta-alpha breakpoints of the two genes occur at different locations within a 35-base pair sequence of exon 3 that is clustered with multiple inverted repeats, whereas the alpha-delta breakpoints reside downstream in two dissimilar blocks of sequences of intron 3. The minimal length of the delta gene sequence that has been replaced by the alpha gene is 55 base pairs in the MiIII gene and 131 base pairs in the MiVI gene. Such segmental DNA transfers may have proceeded unidirectionally through the mechanisms of gene conversion.

Amino Acid Sequence↗
PubMed1990

Minisatellite allele diversification: the origin of rare alleles at the HRAS1 locus.

Three genetic markers within the promoter-exon 1 region of the HRAS1 locus have been employed to investigate lineage relationships among alleles of the highly polymorphic variable tandem repeat (VTR) immediately downstream of the HRAS1 gene. These markers were in absolute linkage disequilibrium with the HRAS1 VTR, allowing the assignment of unique upstream haplotypes to each of the four common VTR alleles. Analysis of 17 rare alleles revealed a stratification of allele fragment size and upstream haplotype in which each rare VTR allele possessed the markers characteristic of the common allele nearest in size. Therefore, hyperallelism emanated from the four common alleles in a defined fashion, the size of a rare allele specifying its origin. As discussed below, this result implies that unequal crossing-over between homologues is unlikely to be the predominant mechanism for generating new VTR alleles at this minisatellite locus.

Alleles↗
PubMed1989

Phenotypic diversification in human neuroblastoma cells: expression of distinct neural crest lineages.

Previous studies of the human neuroblastoma cell line SK-N-SH had demonstrated the presence of and phenotypic interconversion (transdifferentiation) between two morphologically and biochemically distinct cell types: N (neuroblastic) cells with properties of noradrenergic neurons and S (substrate-adherent) cells with properties of melanocytes. Current studies have sought to test the generality of these findings among other cultured human neuroblastoma cell lines and to define further the S-cell phenotype and that of a newly identified, morphologically intermediate, I-type cell. Morphologically homogeneous populations (clonal sublines or subpopulations) of N, S, and I cells were isolated from five additional neuroblastoma cell lines and analyzed biochemically for neuronal, glial, and melanocytic marker enzyme activities and norepinephrine uptake. Immunoblot techniques were used to detect intermediate filament proteins (neurofilament protein, vimentin, glial fibrillary acidic protein) and fibronectin. All N-type cells exhibited neuronal marker enzyme activities, specific uptake of norepinephrine, and presence of one or more neurofilament proteins. S-type cells generally lacked neuronal characteristics but contained, instead, tyrosinase activity (a melanocytic marker enzyme), vimentin, and fibronectin. This combination of attributes is suggestive of a multipotent embryonal precursor cell of the neural crest. I-type cells differentially expressed both S- and N-cell properties and could represent either a stem cell or an intermediate in the transdifferentiation process. Studies of the biological significance of human neuroblastoma cell transdifferentiation and the molecular mechanisms underlying this process may be of relevance to the biological and clinical behavior of this tumor in the patient.

2',3'-Cyclic Nucleotide 3'-Phosphodiesterase↗