A maximum-likelihood phylogenetic tree of host-derived Piggybac transposase sequences. Arrows show the putative recent horizontal gene transfer (HGT) events of host-derived transposable elements. Several transposons are closely related to those from insects. Black boxes indicate elements from the two silkworm-infecting Nosema species, while white boxes indicate elements from the domesticated silkworm B. mori. Numbers in parentheses indicate the total copy numbers for each transposable element.