What explains the gender differences in the adoption of multiple maize varieties? Empirical evidence from Uganda and Tanzania.

Understanding the process that underpins the effective and equitable adoption of modern crop varieties remains an imperative for agricultural development in Africa. This study examines gender differences in adoption rates and determinants of the decision to adopt drought-tolerant (DT) and non-drought-tolerant (ND) maize varieties, based on analysis of maize production data from Tanzania and Uganda. Applying a switching regression with multinomial logit models, we exploit plot level adoption decisions by women and men individually or jointly with in the household, controlling for gender dimension of resource ownership along with other covariates. We find gender differences in the adoption rates for both DT and ND. The results suggest substitution effects noticeable in the decision to use one maize variety rather than another. As the size of the area about which both spouses are jointly owned increases, so does the likelihood that they will adopt DT. Furthermore, DT adoption is also more likely if farmers have experienced frequent climate shocks and dry spells in the past growing seasons. The differences in adoption behavior between men and women jointly or individually are attributable to a combination of the levels and returns of physical- and social-capital factors, as well as to structural issues. One important policy implication of the results is the need for targeted and disaggregated strategies for scaling modern maize varieties, instead of one-size-fits-all approaches.

The dynamics of smallholder farmers' acquisition and distribution of sweetpotato vines in the Lake Victoria Zone Region, Tanzania.

This paper offers new insights into smallholder farmer's practices regarding acquisition and distribution of sweetpotato planting material in the Mwanza and Mara regions of Tanzania by examining three specific issues: (i) farmers' sources of planting material; (ii) factors that influence farmers' sourcing of planting materials outside their own farms and (iii) the types of transactions and social relations involved in farmers' acquisition and distribution of sweetpotato planting material. Data were collected using mixed methods, including a survey of 621 households across nine districts, semi-structured key informant interviews with 28 women sweetpotato farmers, and six focus group discussions. Findings show that farmers in the study area rely almost exclusively on informal seed systems, and that the majority (> 56%) produce their own planting material. Individual, household and community level factors influence farmers' acquisition of planting materials outside their own farms. The sources and mode of transaction related to acquisition/distribution of planting material are strongly influenced by the type of social relationship between the parties involved. Strong social ties facilitate the majority of local planting material acquisitions/distributions, and favor provision of locally available planting material as a gift/without payment.Weak social ties are primarily associated with the transaction modality of purchase/sale, and frequently help facilitate acquisition of new or exotic planting material. The findings provide entry points both for entities that seek to enhance small-scale farmers' access to improved, high quality sweetpotato germplasm, as well as broader efforts to strengthen research and development strategies for integrating formal and informal seed systems.

Structural and functional constraints limit options for cytotoxic T-lymphocyte escape in the immunodominant HLA-B27-restricted epitope in human immunodeficiency virus type 1 capsid.

Control of human immunodeficiency virus type 1 (HIV-1) by HLA-B27-positive subjects has been linked to an immunodominant CD8(+) cytotoxic T-lymphocyte (CTL) response targeting the conserved KK10 epitope (KRWIILGLNK(263-272)) in p24/Gag. Viral escape in KK10 typically occurs through development of an R(264)K substitution in conjunction with the upstream compensatory mutation S(173)A, and the difficulty of the virus to escape from the immune response against the KK10 epitope until late in infection has been associated with slower clinical progression. Rare alternative escape mutations at R(264) have been observed, but factors dictating the preferential selection of R(264)K remain unclear. Here we illustrate that while all observed R(264) mutations (K, G, Q, and T) reduced peptide binding to HLA-B27 and impaired viral replication, the replicative defects of the alternative mutants were actually less pronounced than those for R(264)K. Importantly, however, none of these mutants replicated as well as an R(264)K variant containing the compensatory mutation S(173)A. In assessing the combined effects of viral replication and CTL escape using an in vitro coculture assay, we further observed that the compensated R(264)K mutant also displayed the highest replication capacity in the presence of KK10-specific CTLs. Comparisons of codon usage for the respective variants indicated that generation of the R(264)K mutation may also be favored due to a G-to-A bias in nucleotide substitutions during HIV-1 replication. Together, these data suggest that the preference for R(264)K is due primarily to the ability of the S(173)A-compensated virus to replicate better than alternative variants in the presence of CTLs, suggesting that viral fitness is a key contributor for the selection of immune escape variants.

Escape from the dominant HLA-B27-restricted cytotoxic T-lymphocyte response in Gag is associated with a dramatic reduction in human immunodeficiency virus type 1 replication.

Human leukocyte antigen (HLA)-B27-positive subjects are uncommon in their ability to control infection with human immunodeficiency virus type 1 (HIV-1). However, late viral escape from a narrowly directed immunodominant Gag-specific CD8(+) T-lymphocyte (CTL) response has been linked to AIDS progression in these individuals. Identifying the mechanism of the immune-mediated control may provide critical insights into HIV-1 vaccine development. Here, we illustrate that the CTL escape mutation R(264)K in the HLA-B27-restricted KK10 epitope in the capsid resulted in a significant defect in viral replication in vitro. The R(264)K variant was impaired in generating late reverse transcription products, indicating that replication was blocked at a postentry step. Notably, the R(264)K mutation was associated in vivo with the development of a rare secondary mutation, S(173)A, which restored viral replication in vitro. Furthermore, infectivity of the R(264)K variant was rescued by the addition of cyclosporine A or infection of a cyclophilin A-deficient cell line. These data demonstrate a severe functional defect imposed by the R(264)K mutation during an early step in viral replication that is likely due to the inability of this variant to replicate efficiently in the presence of normal levels of cyclophilin A. We conclude that the impact of the R(264)K substitution on capsid structure constrains viral escape and enables long-term maintenance of the dominant CTL response against B27-KK10, providing an explanation for the protective effect of HLA-B27 during HIV infection.