Identification and infectivity of different microsporidian isolates collected from sericulture (Bombyx mori L) areas

Abstract

Pebrine is the most destructive disease of silkworm, Bombyx mori L., as it is transmitted transovarially to the next generation and causes crop loss. Pebrine disease is caused by microsporidian strains whose pathogenicity, rate of spread, and mode of transmission differ. This has recently become a serious concern for seed producers in India. The threat has been increasing, and disease incidence has increased in various locations in Karnataka and Tamil Nadu. A disease survey was conducted at various locations in these two states to record disease incidence rates. Ten microsporidian isolates were collected and inoculated for bioassay studies into healthy larvae to determine their infectivity, multiplication, and transovarial transmission. Rearing was conducted for two successive generations, and the mortality rate in the second generation varied for all the microsporidian isolates, with pupal survival ranging from 21-65%. All ten isolates of microsporidia were found to exhibit transovarial transmission. The fecundity significantly decreased and the hatching percentage decreased from 5% to 22% in the second generation of infected batches. The presence of microsporidia was confirmed through PCR amplification of the SSU rRNA gene of Microsporidia. Five microsporidian isolates were selected for gene sequence analysis on the basis of their collection at different locations and identified as being homologous to Nosema plutellae, Nosema fumiferenae and Nosema species. This study helps in understanding the infectivity levels of different microsporidia prevalent in the sericulture field.