DNA distribution in chromosomes ofLathyrus species

DNA content in South American endemic species of Lathyrus

The genome size was surveyed in 13 Notolathyrus species endemic to South America by flow cytometry and analyzed in an evolutionary and biogeographic context. A DNA content variation of 1.7-fold was registered, and four groups of species with different DNA content were determined. Although, the 2C values were correlated with the total chromosome length and intrachromosomal asymmetry index (A1), the karyotype formula remained almost constant. The conservation of the karyotype formula is in agreement with proportional changes of DNA in the chromosome arms. Species with annual life cycle and shorter generation time had the lowest DNA content and the data suggest that changes in DNA content involved reductions of genome size in the perennial to annual transitions. The variation of 2C values was correlated with precipitation of the coldest quarter and, to some extent, with altitude. Additional correlations with other variables were observed when the species were analyzed separately according to the biogeographic regions. In general, the species with higher DNA content were found in more stable environments. The bulk of evidence suggests that changes on genome size would have been one of the most important mechanisms that drove or accompanied the diversification of Notolathyrus species.

Karyotype analysis and chromosome evolution in species of Lathyrus (Fabaceae)

The karyotypes of 21 accessions of Lathyrus L. belonging to four sections were investigated. Although all the species have a chromosome number of 2n = 14, they could be differentiated by their karyotype formula and quantitative parameters of the karyotypes. Phenetic distance showed that in spite of the differences observed among entitied, they can be grouped in clusters that coincide with the taxonomic sections established by Kupicha and with the life cycle of the species. The section Clymenum can be distinguished by the presence of a subtelocentric pair. From an evolutionary point of view, variation in genome size, however, is congruent with morphological variation as well as with the life cycle.

DNA content, rDNA loci, and DAPI bands reflect the phylogenetic distance between Lathyrus species

The nuclear DNA content, the proportion of A+T base pairs, the chromosomal positions of 5S and 25S rRNA genes, as well as of DAPI (4,6-diamidino-2-phenylindole) bands are described for seven species belonging to three different sections of the genus Lathyrus. These data and chromosome measurements allowed to establish precise idiograms and to discriminate most of the chromosome pairs of the seven species. The karyotypic features correlate well with the phylogenetic distances between these species.

Karyotype morphology and evolution in some Lathyrus (Fabaceae) species of southern Brazil

The karyotypes of Lathyrus nervosus Lam., L. pubescens Hook. et Arn., L. paranensis Burk. and L. crassipes Gill ap. Hook et Arn., native to Rio Grande do Sul (southern Brazil), are described in detail for the first time. All taxa have 2n = 14 chromosomes. The karyotypic formulae were 2 m + 12 sm for L. nervosus, L. pubescens and L. paranensis and 4 m + 10 sm for L. crassipes. In all species, the smallest chromosome pair bore a secondary constriction with a satellite in the long arm. Intraspecific variability in the position and number of secondary constrictions was observed in L. nervosus and L. pubescens. All of the species had a conservative and similar karyotype morphology, but differed in total complement size by as much as 20% between the highest (L. nervosus) and lowest (L. crassipes) values. These results suggest that changes in chromosome size during evolution have been similar for all the chromosomes of the complement. Together with data on the life cycle and mode of reproduction, these results also indicate that L. crassipes is a derived taxon, if an evolutionary trend towards a decrease in chromosome size is accepted.

Evolutionary implications of the relationship between genome size and body size in flatworms and copepods

Genome and body sizes were measured in 38 species of turbellarian flatworms and 16 species of copepod crustaceans. Significant positive relationships existed between genome size and body size in both groups. The slopes of these regressions indicated that increases in cell volume are reinforced by increased cell numbers, or that cell volumes show positive allometric variation with genome size. Genome sizes appear to vary in a discontinuous fashion among congeneric species in both groups, indicating that such changes have occurred rapidly, and with potentially profound effects on important morphological characters.

Chromosomal characterization of three native and one cultivated species of Lathyrus L. in Southern Brazil

Mitotic metaphase chromosomes and interphase nuclei of nine populations of three South American species of Lathyrus (L. pubescens, L. nervosus and L. crassipes) and six populations of the cultivated species L. odoratus were analyzed. All populations had 2n = 2x = 14 chromosomes. There were significant differences among populations within each species and among species in the number of metacentric, submetacentric and subtelocentric chromosomes, the number and location of secondary constrictions, chromosome length (longest and shortest), total haploid complement, arm ratio, and centromeric index. L. odoratus showed the highest tendency towards karyotype symmetry whereas the three South American species showed a moderate tendency towards asymmetry, with L. pubescens being the most asymmetrical. Silver staining was used to identify the nucleolar organizer regions (NORs) and the number of nucleoli per interphase nucleus in each species. In L. pubescens and L. nervosus, the NORs were located on the secondary constriction of the long arm of pair 7, in L. crassipes, the NOR was proximal being located in the pair of metacentric chromosomes, and in L. odoratus there were four terminal NORs on the short arms of pairs 4 and 5. The four species had a maximum of four nucleoli per interphase nucleus, indicating the presence of four regions with active ribosomal genes in each case.

The Modulation of DNA Content: Proximate Causes and Ultimate Consequences

The forces responsible for modulating the large-scale features of the genome remain one of the most difficult issues confronting evolutionary biology. Although diversity in chromosomal architecture, nucleotide composition, and genome size has been well documented, there is little understanding of either the evolutionary origins or impact of much of this variation. The 80,000-fold divergence in genome sizes among eukaryotes represents perhaps the greatest challenge for genomic holists. Although some researchers continue to characterize much variation in genome size as a mere by-product of an intragenomic selfish DNA “free-for-all” there is increasing evidence for the primacy of selection in molding genome sizes via impacts on cell size and division rates. Moreover, processes inducing quantum or doubling series variation in gametic or somatic genome sizes are common. These abrupt shifts have broad effects on phenotypic attributes at both cellular and organismal levels and may play an important role in explaining episodes of rapid—or even saltational—character state evolution.

Nuclear architecture in plants

Structure within the nucleus of plants is becoming increasingly clear in both metaphase and interphase nuclei, although there are conflicting data about the relative positions of individual and pairs of chromosomes. At interphase, individual chromosomes may generally occupy discrete domains that are not intermixed with other chromosomes. Aspects of mechanical chromosome behaviour and even of gene expression may correlate with interphase chromosome position, and imply that a better understanding of nuclear architecture is required.

Changes in DNA composition in the evolution of Vicia species

The composition of nuclear DNA in 3 Vicia species are compared. The species V. eriocarpa, V. johannis and V. melanops are from three separate subgeneric sections of Vicia and show a fourfold variation in their amounts of nuclear DNA. DNA melting experiments, buoyant density gradient analysis and Cot reassociation experiments show that the quantitiative change in nuclear DNA between the three species is achieved by changes in the amounts of both repetitive and nonrepetitive DNA sequences. It is suggested that while the increase in the repetitive fraction is achieved by the proliferation of repetitive base sequences the increase in the nonrepetitive fraction is due to the steady accretion of highly diverged base sequences resulting from mutations, deletions, insertions and base sequence rearrangements among families of repetitive sequences.