Frequência de hermafroditas e distribuição de tipos de acasalamento em populações de Fusarium verticillioides associadas ao milho em diferentes zonas climáticas do Brasil

Abstract

Brazil is one of the biggest producers and consumers of maize (Zea mays L.) in the world and the fusariosis caused by F. verticillioides, on ears and or on stalk, is a recurrent problem when environmental conditions favor epidemics in the field or in storage. The present study aimed to characterize through crosses the F. verticillioides isolates from the three main climatic zones where maize is grown in Brazil. The fertility and effective population size were calculated from data generated by crossing field isolates with female fertile testers of F. verticillioides. For the entire population, 231 out of 300 isolates were cross-fertile with tester isolates. MAT-1 and MAT-2 idiomorphs of the fertile isolates segregated in a 105:126 rate. Female isolates (hermaphrodites) were 96 out of 231 fertile isolates, while 135 were male only isolates. The Ne(mt) was 99% of the count for the Brazilian population when the mating type idiomorphs were used as predictors in the estimative of the population size. But when this calculation was made on the basis of the female fertile isolates the Ne(f) was 83%. When the total population is divided into sub-populations representing climatic zones, the frequencies of female fertile isolates were 33 out of 80 fertile isolates from the Equatorial Tropical Zonal Climate (ETZC); 14 out of 78 fertile isolates from the Temperate Zonal Climate (TZC); and 49 out of 73 fertile isolates from the Tropical Central Brazil Zonal Climate (TCBZ). The effective population size expressed in percentage of the total number of individual in each population for the three populations represented by isolates from the ETZC, TZC and TCBZ were Ne(mt) = 99, 100, 97 and Ne(f) = 83, 52, 96 respectively. The Brazilian population of F. verticillioides shows high fertility and high effective population size, suggesting the possibility of frequent sexual reproduction in the field. Apparently there is a tendency of higher fertility in populations established at low latitudes, represented by the tropical climate.