Estimates of genetic parameters of certain characters in Hevea brasiliensis

Heritability estimates of five quantitative characters, namely, yield, girth, girth increment, virgin bark and renewed bark thickness, of the breeding Phase III Hevea families have been obtained by variance component analyses. In general, the combined heritability estimates for various characters were low to moderate. The heritabilities of these characters based on female variance components, however, were high, suggesting that considerable improvement of each of the characters could be achieved in properly designed experiments.Estimates of heritability for average yields (Range: 0.11-0.34) over different years showed that the first three years' yield was adequate for predicting estimates of genetic variance for the average of five years' yield.Correlation studies on yield with other characters indicated considerable influence of environment, with genetic correlations accounting for about 0.07 to 0.36 in the characters studied.Expected direct response to selection in yield and correlated response in yield to selection for girth at opening and virgin bark thickness have been calculated using three arbitrary values of selection intensity. The efficiency of the correlated response was found to be approximately half that of the direct response. However, the indirect selection for yield using virgin bark thickness appeared to be more favourable than that using the girth at opening favoured by earlier workers.

MUTATION SPECTRUM IN THE FUNGUS CONIOCHAETA VELUTINA

Auxotrophic, morphological, sensitive and suppressor mutant strains of Coniochaeta velutina are described. Of 1334 auxotrophic mutants isolated, all but 58 have been characterized as to their specific requirements. The mutation spectrum obtained on malt medium differs in several respects from that on complete medium. Arginine, adenine, cysteine, methionine, histidine and pyrimidine requiring mutants were most frequently isolated. Of 12 diauxotrophs tested genetically, 7 proved to be double mutants, but the remaining 5 behaved genetically as single mutants with a double requirement. Four of these required both adenine and histidine for growth and the other both phenylalanine and tyrosine. Cysteine-sensitive mutants and histidine-sensitive strains are described. Other mutants isolated included a variety of morphological, conidial and colour variants, UV-sensitive strains, canavanine-sensitive strains and arginine-suppressor mutations.

An introduction to the genetics of the fungus Coniochaeta veluitina

The fungus Coniochaeta velutina (Fuckel) Munk., which inhabits defective xylem in maple trees, has been developed for a joint genetical and plant pathological program. This paper describes progress made in the genetical aspect of the problem. From the more than 80 mutants forms which have been isolated after ultraviolet irradiation, 34 have been analyzed genetically by random spore analysis. Many of the mutant strains were incapable of acting as protoperithecial parents in crosses, thus limiting the number of crosses which could be made. Three linkage groups have been discovered, but many of the genes investigated showed no linkage with any other. This finding is in keeping with similar genetic analyses with other fungi.Linkage between the two pyrimidine-requiring mutants in Coniochaeta parallels a similar finding in Neurospora.