Dosage and parent-of-origin effects shaping aneuploid swarms in A. thaliana.

Changes in chromosome number have a critical role in the evolution and formation of plant species. Triploids, which carry three complete sets of chromosomes, in particular produce offspring with different chromosome numbers, including diploid and tetraploid progeny, as well as a swarm of aneuploid progeny, which carry incomplete chromosome sets. In this study, we investigated the mechanisms shaping these swarms at the population level through a detailed characterization of the progeny of triploid Arabidopsis thaliana. We report that triploid meiosis predominately produced aneuploid gametes, most of which were viable. We performed reciprocal crosses between triploid and either diploid or tetraploid plants and karyotyped all surviving individuals. This allowed us to dissect the parent-of-origin (cross-direction) effects and also the effect of the dosage of the crossing partner on the inheritance of each chromosome type. Overall, our data indicate that the chromosomal composition of the swarms produced by the triploid A. thaliana were strongly influenced by selection acting against specific gamete combinations, but not necessarily associated with aneuploidy. Finally, each of the five chromosome types responded differently to this selection, suggesting the presence of dosage-sensitive factor(s) critical for viability and encoded on different chromosomes.

Phenotypic instability and rapid gene silencing in newly formed arabidopsis allotetraploids.

Allopolyploid hybridization serves as a major pathway for plant evolution, but in its early stages it is associated with phenotypic and genomic instabilities that are poorly understood. We have investigated allopolyploidization between Arabidopsis thaliana (2n = 2x = 10; n, gametic chromosome number; x, haploid chromosome number) and Cardaminopsis arenosa (2n = 4x = 32). The variable phenotype of the allotetraploids could not be explained by cytological abnormalities. However, we found suppression of 20 of the 700 genes examined by amplified fragment length polymorphism of cDNA. Independent reverse transcription-polymerase chain reaction analyses of 10 of these 20 genes confirmed silencing in three of them, suggesting that approximately 0.4% of the genes in the allotetraploids are silenced. These three silenced genes were characterized. One, called K7, is repeated and similar to transposons. Another is RAP2.1, a member of the large APETALA2 (AP2) gene family, and has a repeated element upstream of its 5' end. The last, L6, is an unknown gene close to ALCOHOL DEHYDROGENASE on chromosome 1. CNG DNA methylation of K7 was less in the allotetraploids than in the parents, and the element varied in copy number. That K7 could be reactivated suggests epigenetic regulation. L6 was methylated in the C. arenosa genome. The present evidence that gene silencing accompanies allopolyploidization opens new avenues to this area of research.

Inheritance of resistance to the cowpea aphid in cowpea.

Inheritance of resistance to cowpea aphid, Aphis craccivora Koch, in three resistant cultivars of cowpea, Vigna unguiculata (L.) Walp, was studied. The parents, F1 and F2 population were grown in an insect-proof screenhouse. Each 3-day-old seedling was infested with 10 apterous adult aphids. Seedling reaction was recorded when the susceptible check was killed. The segregation data revealed that the resistance of ICV11 and TVU310 is governed by single dominant genes. All the F2 seedlings of the cross ICV10xTVU310 were resistant, indicating that they have the same gene for resistance. However, the F2 populations from the crosses ICV10xICV11 and ICV11xTVU310 segregated in a ratio of 15∶1, indicating that the dominant genes in ICV11 and TVU310 are non-allelic and independent of each other. The resistance gene of ICV10 and TVU310 is designated as Ac1 and that of ICV11 as Ac2.

Correlation studies on yield and fibre traits in upland cotton (Gossypium hirsutum L.).

Two diverse parents of upland cotton namely J.34 and I.C. 1926 were crossed. A comparison between biparental intermated progenies and F3 families indicated alteration of correlation coefficient between yield and halo length. The significant negative correlation in F3 population between these two attributes changed to a positive but non significant one in biparental intermated progenies. A change in correlation coefficients was expected due to breakage of linkage upon intermating. An increase in the correlation coefficients could also be expected when linkages are predominantly in the repulsion phase. It is suggested that intermating in early generations coupled with selection of desirable segregants may prove a useful method for improving yield and quality simultaneously. The diallel selective mating system may also supplement intermating to improve yield and quality in cotton.