Index of postzygotic reproductive isolation for reciprocal cross data (IPO). V/F, viable and/or fertile; I, inviable or sterile.

(A) Both sexes viable or fertile in both reciprocal crosses; (B) one sex sterile or inviable in one cross only; (C) one sex inviable or sterile in both directions of cross; (D) both sexes inviable or sterile in only one direction of cross; (E) one sex viable or fertile in both directions of cross; (F) both sexes inviable or sterile in both directions of cross; M = males; F = females.