Complete genome sequencing of two causative viruses of cassava mosaic disease in Ghana.

Cassava mosaic disease (CMV), caused by one or a combination of cassava mosaic geminiviruses, is ranked among the most important constraints to profitable and efficient production of cassava. Effective control measures require in-depth knowledge of the viral causative agent. Using rolling-circle amplification and unique enzymes, the full genome of two species of cassava mosaic geminivirus isolated from infected cassava plants in Ghana were cloned into pCambia 1300 and pET-28b. The sequences of the genome were determined on an ABI sequencer and a pairwise comparison was performed with other cassava-infecting geminiviruses from different countries. It was revealed that cassava grown in Ghana is attacked by two species of geminivirus in either single or mixed infections. These are the African cassava mosaic virus (ACMV) and the East African cassava mosaic virus (EACMV)-like, with high sequence similarity of 94% and 80%, respectively, between the DNA-A and DNA-B components of each virus, and 66% and 41% similarity of the common region (CR) (for A and B accordingly). The DNA-A of ACMV and EACMV-like contained 2781 and 2800 nucleotides, respectively, while their DNA-B components had 2725 and 2734 nucleotides, respectively. ACMV DNA-A was over 97% similar to those of other ACMVs from the continent. In contrast, EACMV-like DNA-A was over 98% similar to the isolates from Cameroon and other West African countries, and less than 88% similar to other EACMV species. Thus ACMV and EACMV-like were named African cassava mosaic virus-Ghana and East African cassava mosaic Cameroon virus-Ghana. Computer analysis revealed that their genome arrangement follows the typical old world bipartite begomovirus genome. The association of these two species and their interaction might account for the severe symptoms observed on infected plants in the field and in the greenhouse.

Genetic structure of seventy cocoyam (Xanthosoma sagittifolium, Linn, Schott) accessions in Ghana based on RAPD.

Seventy cocoyam accessions collected from the eastern and Volta regions of Ghana were studied using RAPD technique. Ten primers were used to study the genetic diversity and structure of the experimental material and a total of 120 different bands were detected. Levels of polymorphic fragments detected by the ten primers ranged from 69.2% to 100%. In the accessions from the eastern region, the mean observed and effective number of alleles per individual per population and mean Nei's gene diversity were 1.99+/-0.01, 1.53+/-0.12 and 0.312+/-0.05, respectively. Mean observed and effective number of alleles per individual per population and Nei's gene diversity were 1.76+/-0.17, 1.44+/-0.16 and 0.264+/-0.08, respectively for the accessions from the Volta region. Considering the entire collections, the mean observed and effective number of alleles per individual per population and Nei's gene diversity for primers were 1.99+/-0.02, 1.54+/-0.12 and 0.319+/-0.05, respectively. In terms of Nei's F-statistics in the subdivided populations for all primers, the overall gene diversity (Ht) ranged from 0.230 to 0.396 with a mean of 0.313+0.06, within sample gene diversity (Hs) ranged from 0.214 to 0.372 with a mean of 0.286+0.06, gene differentiation (Gst) ranged between 0.005 and 0.178 with a mean of 0.086. Geneflow estimate ranged between 2.309 and 99.500 with a mean of 5.314. The accessions grouped into three main clusters. Accession BD96/183 was the most diverse and may be incorporated into cocoyam breeding programs. The 70 accessions did not cluster into their distinct geographical regions suggesting that there may have been movement of germplasm across the two regions.

Evidence of natural selection for disease resistance in Composite Cross Five (CCV) of barley.

Cambridge Composite Cross Five (CCV) of barley was studied utilising hordeins, restriction fragment length polymorphisms (RFLPs) and reaction to powdery mildew with a view to understanding the genetic changes occurring in the population. Changes in the frequency of individual hordein patterns as well as pattern combinations showed directional trends in successive generations in three parallel populations maintained as discrete populations since 1977 in Cambridge. Certain hordein pattern combinations were more common in the resistance classes and there was a strong association between hordein patterns and mildew reaction. RFLP analysis revealed that 80% of a random sample taken from generation F24 of Population I had the same restriction pattern as that of the cultivar Algerian, which was one of the original 30 parental lines of CCV. This cultivar is the source of the Mla1 allele in barley improvement programmes in Europe. We argue, based on supporting evidence from hordein analysis and tests of reaction to selected mildew isolates of known virulence isolates together with UK virulence surveys, that selection for Mla1 in Cambridge has been the predominant evolutionary force in CCV in Cambridge.

Hordein variation and reaction to powdery mildew in composite cross XLII of barley.

A bulk hybrid population, CCXLII was investigated for hordein variation and reaction to powdery mildew. The results indicated that the population in F4 was genetically variable and contained an appreciable proportion of heterozygotes. Evidence was found for differential viability within families. This was possibly the result of a high segregation load. The pattern of genetic variation suggests that although the population could be a useful source of breeding material for the selection of new lines, it may be risky as a method of conservation of germplasm.