Since theBombyx morigenome sequence was published, conserved synteny betweenB. moriand some
other lepidopteran species has been revealed by either FISH (fluorescencein situhybridization) with BAC
(bacterial artificial chromosome) probes or linkage analysis. However, no species belonging to the Noctuidae, the largest lepidopteran family which includes serious polyphagous pests, has been analyzed so far
with respect to genome-wide conserved synteny and gene order. For that purpose, we selected the noctuid
speciesHelicoverpa armigeraandMamestra brassicae, both withn¼31 chromosomes. Gene-defined fosmid
clones fromM. brassicaeand BAC clones from a closely related species ofH. armigera, Heliothis virescens,
were used for a FISH analysis on pachytene chromosomes. We recognized allH. armigerachromosomes
from specific cross-hybridization signals of 146 BAC probes. With 100 fosmid clones we identified and
characterized all 31 bivalents ofM. brassicae. Synteny and gene order were well conserved between the two
noctuid species. The comparison with the model speciesB. mori(n¼28) showed the same phenomenon for
25 of the 28 chromosomes. Three chromosomes (#11, #23 and #24) had two counterparts each in
H. armigeraandM. brassicae. Since n¼31 is the modal chromosome number in Lepidoptera, the noctuid
chromosomes probably represent an ancestral genome organization of Lepidoptera. This is the first
identification of a full karyotype in Lepidoptera by means of BAC cross-hybridization between species. The
technique shows the potential to expand the range of analyzed species efficiently.