Nuclear variation withinAvenae

Genome size variation in the genus Avena

Genome size is an indicator of evolutionary distance and a metric for genome characterization. Here, we report accurate estimates of genome size in 99 accessions from 26 species of Avena. We demonstrate that the average genome size of C genome diploid species (2C = 10.26 pg) is 15% larger than that of A genome species (2C = 8.95 pg), and that this difference likely accounts for a progression of size among tetraploid species, where AB < AC < CC (average 2C = 16.76, 18.60, and 21.78 pg, respectively). All accessions from three hexaploid species with the ACD genome configuration had similar genome sizes (average 2C = 25.74 pg). Genome size was mostly consistent within species and in general agreement with current information about evolutionary distance among species. Results also suggest that most of the polyploid species in Avena have experienced genome downsizing in relation to their diploid progenitors. Genome size measurements could provide additional quality control for species identification in germplasm collections, especially in cases where diploid and polyploid species have similar morphology.

Chromosomal and genomic organization of Ty1-copia-like retrotransposon sequences in the genus Avena

A cloned repetitive sequence, pAvKB30, obtained from an Avena vaviloviana (AB genome) genomic library, along with two polymerase chain reaction products derived from the conserved region of the reverse transcriptase (RT) gene of retrotransposons, were characterized molecularly and cytologically. The cloned DNA fragment was a dispersed repeat present in all Avena species used in this study (A. strigosa, A. clauda, A. vaviloviana, A. magna, and A. sativa). The fragment was sequenced (210 bp) and found to be 69.5% homologous to part of WIS-2-1A, and 60.5% homologous to the leader sequence of BARE-1; both of these elements have been characterized as Ty1-copia-like retrotransposons in wheat and barley, respectively. In situ hybridization of pAvKB30 to diploid, tetraploid, and hexaploid oat species revealed that the probe is present on both arms of all chromosomes (A, B, C, and D genomes) but is excluded from their centromeric and nucleolar organizer regions. By using double in situ hybridization in hexaploid A. sativa (ACD genome), pAvKB30 was found to be present in lower copy numbers in C-genome chromosomes compared with A- and D-genome chromosomes. Furthermore, under low stringency conditions, pAvKB30 hybridized on Southern blots containing barley, wheat, rye, and Arrhenatherum DNA. However, under high stringency conditions, it hybridized only on Arrhenatherum DNA, which is considered to be the genus most closely related to Avena. All Avena species included in this study yielded a PCR product when the primers from the RT domain of retrotransposons were used. Two products, rtA, obtained by using A. strigosa (A(s) genome) as template, and rtC, obtained by using A. clauda (Cp genome) as template, gave Southern and in situ hybridization results similar to pAvKB30, but each was more abundant in its genome of origin.

Cytophotometric and biochemical analyses of DNA in pentaploid and diploid Agave species

Nuclear DNA content, chromatin structure, and DNA composition were investigated in four Agave species: two diploid, Agave tequilana Weber and Agave angustifolia Haworth var. marginata Hort., and two pentaploid, Agave fourcroydes Lemaire and Agave sisalana Perrine. It was determined that the genome size of pentaploid species is nearly 2.5 times that of diploid ones. Cytophotometric analyses of chromatin structure were performed following Feulgen or DAPI staining to determine optical density profiles of interphase nuclei. Pentaploid species showed higher frequencies of condensed chromatin (heterochromatin) than diploid species. On the other hand, a lower frequency of A-T rich (DAPI stained) heterochromatin was found in pentaploid species than in diploid ones, indicating that heterochromatin in pentaploid species is made up of sequences with base compositions different from those of diploid species. Since thermal denaturation profiles of extracted DNA showed minor variations in the base composition of the genomes of the four species, it is supposed that, in pentaploid species, the large heterochromatin content is not due to an overrepresentation of G-C repetitive sequences but rather to the condensation of nonrepetitive sequences, such as, for example, redundant gene copies switched off in the polyploid complement. It is suggested that speciation in the genus Agave occurs through point mutations and minor DNA rearrangements, as is also indicated by the relative stability of the karyotype of this genus. Key words : Agave, DNA cytophotometry, DNA melting profiles, chromatin structure, genome size.

Meiotic variation in an intergenomic autopolyploid series. I. Chiasma frequency

Variation in chiasma frequency has been studied in PMCs of diploids and C0 autotetraploids of seven Lathyrus species exhibiting a range of genome size (10.8-19.9 pg DNA/2C). Variation in chiasma frequency showed no relation to changes in genome size, either between species or between disomic sets within nuclei. Mean chiasma frequency of the tetraploids showed a 75% increase over that in the diploids. Half of this increase represents an additive effect of chromosome doubling. Total variance in chiasma frequency of autotetraploids increases by 80% over that in diploids, in line with the square of the multiplicative effect of chromosome doubling. At the diploid level, interspecific differences account for the major component of variance (63.1%). Phenotypic variation in chiasma frequency was apparent in all seven species but represented the smallest component of variance (2.8%). Chromosome doubling results in an eightfold increase in the absolute size of the phenotypic component of variance in chiasma frequency and a threefold increase in that of the cellular component. It has no effect on the absolute size of the interspecific component.

Nuclear dna amounts in angiosperms

The number of angiosperm species for which nuclear DNA amount estimates have been made has nearly trebled since the last collected lists of such values were published, and therefore, publication of a more comprehensive list is over due. This paper lists absolute nuclear DNA amounts for 753 angiosperm species. The dats were assembled primarily for reference purposes, and so the species are listed in alphabetical order, as this was felt to be more helpful to cyto- and biochemists whom, it is anticipated, will be among its major users. The paper also reviews aspects of the history, nomenclature, methods, accuracy and problems of nuclear DNA estimation in angiosperms. No attempt is made to reconsider those aspects of nuclear DNA estimation which have been fully revised previously, although the bibliography of such aspects is given. Instead, the paper is intended as a source of basic information regarding the terminology, practice and limitations of nuclear DNA estimation, especially by Feulgen microdensitometry, as currently practiced.