The origin of anholocyclic, parthenogenetic species of Adelgidae on the basis of a study of life-cycle forms and their cytology.
Observations in the Oldenburg region showed infestations sometimes by the holocyclic Gilletteella (Adelges) cooleyi, causing damage to both Sitka Spruce as primary and Douglas Fir as secondary host, and sometimes by a morphologically indistinguishable anholocyclic aphid that did not cause galls on Sitka Spruce (transferred by the author from G. cooleyi var. coweni to specific rank as G. coweni). Rearing experiments and cytological analyses showed that G. cooleyi has two parallel cycles, one male, leading to andropara and sexualis male, and one female, leading to gynopara and sexualis female, the female cycle only giving rise to a paracycle of progredientes from hiemosistentes. The two cycles differed in their chromosome sets, the paracycle conforming to the female type. The chromosome set of G. coweni was identical with that of the female cycle of G. cooleyi, suggesting that G. coweni originated by a mutation eliminating the male cycle. G. coweni, however, still has a gynopara-sexualis female sequence on Sitka Spruce that does no damage. Parallel findings on other holocyclic and anholocylic adelgids including A. laricis, A. viridis, Pineus pini, Eopineus strobi, E. pineoides, A. piceae and A. viridana suggest that this is a common mode of evolution in the group.KEYWORDS: Adelges Chermes, Dreyfusia, Gilletteella, Sacchiphantesspp cooleyi \ Adelges Chermes, Dreyfusia, Gilletteella, Sacchiphantesspp coweni \ Adelges Chermes, Dreyfusia, Gilletteella, Sacchiphantesspp laricis \ Adelges Chermes, Dreyfusia, Gilletteella, Sacchiphantesspp piceae \ Adelges Chermes, Dreyfusia