Serological and biological variations of African cassava mosaic virus, in Nigeria

To determine the occurrence of variants of African cassava mosaic virus, 316 cassava leaf samples were collected from mosaic‐affected cassava plants in 254 farmers. fields in 1997 and 1998, covering the humid forest, coastal/derived, southern Guinea and northern Guinea savannas and arid and semi‐arid agroecologies of Nigeria. The samples were tested in triple antibody sandwich enzyme‐linked immunosorbent assay using a panel of 10 monoclonal antibodies (MAbs) against the virus in which 29 reaction patterns were observed. In cluster analysis, nine serotypes were obtained at 0.80 Jaccard similarity coefficient index in which at least 50% of isolates of each serotype reacted alike. The serotypes ranged between two extremes: serotype 1 with 90% isolates reacting with the 10 MAbs and serotype 8 in which 90% of its isolates failed to react with the antibodies. Isolates of serotypes 1, 2, 4 and 8 were widely distributed while those of the other serotypes were estricted to certain agroecologies. Four representative isolates 227 (serotype 1), 231 (serotype 2), 235 and 283 (serotype 8) elicited different responses in Nicotiana, benthamiana, with isolate 283 not able to infect this and other test plants used. The serological variations did not necessarily reflect the biological variations. In polymerase chain reaction tests, one out of the five pairs of ACMV primers tested distinguished only isolate 283. The humid forest, derived/coastal and southern Guinea savannas where most of the crop is grown in Nigeria had a high number of variants, which makes the agroecologies suitable for the selection of resistant cassava clones against ACMV.     

Physiological mechanism governing slow and fast development in predatory ladybirds

Aphidophagous and coccidophagous ladybirds, similar to their prey, show marked differences in their pace of life (Dixon, 2000), in particular in their rate of development, with all stages of aphidophagous species developing much faster than those of coccidophagous species. Two hypotheses are proposed to account for the large difference in the pace of life of these two groups. These are that differences in the rate of development are a result of differences in lower temperature thresholds for development or the quality of their respective prey as food (Dixon et al., 2011). Analysis of published results on the rates of development of the eggs of ladybirds indicates that the inverse relationships between the number of day‐degrees required for development (K) and the lower temperature threshold for development (td_min) of these two groups are significantly different. In particular, the respective td_min overlap and K of the aphidophagous and coccidophagous species with a similar td_min are, on average, 38 and 117 day‐degrees (D^o). The relationship between the rate of development (R) and temperature (T) for aphids reared on poor‐ or high‐quality foods indicates that, although the value of td_min of a species depends on food quality, K does not, showing that it is unlikely that K is governed by food quality. Thus, there is little support for differences in either the td_min or food quality governing the difference in the pace of life of these two groups of ladybirds. The results indicate that the physiological mechanism that may govern the difference in the pace of life between these two groups is the number of day‐degrees (K) needed to complete their development. The possible evolutionary reason for this is discussed.