Pemphigus populitransversus

The North American aphid Pemphigus populi-transversus Riley (Hemiptera: Aphididae: Pemphiginae) forms galls on the leaves of the eastern cottonwood (Populus deltoides; Salicaceae) throughout the eastern and central United States. From the 1940s to the 1990s, Dr. Robert Sokal and his associates published extensively on character and geographical variation in P. populi-transversus in the central and eastern United States (citations summarized in Sokal et al. 1991). One of their more intriguing observations was that most populations surveyed exhibited two distinct gall types, which they named ‘‘elongate’’ and ‘‘globular,’’ based on their shapes (Bird et al. 1979; Sokal and Riska 1981). The aphids forming the two types differed at several allozyme loci and in the seasonal production of sexuals, with the sexuals of the elongate-type appearing in the fall and those of the globular-type appearing in the spring (Faith 1979a,b; Rhomberg 1980; Setzer 1980). Faith (1979a,b) hypothesized that the unusual gall polymorphism could reflect these underlying differences in life cycles, suggesting a rare case of incipient allochronic speciation.

The relationship between P. populi-transversus and a closely related aphid species, P. obesinymphae Aoki, has been of the most confusing among North American Pemphigus species (Aoki and Moran 1994; Blackmon and Eastop 1994). Pemphigus obesinymphae was originally described in the southwestern United States on Populus fremontii (a cottonwood that is likely the sister taxon of P. deltoides; Eckenwalder 1996). Pemphigus obesinymphae was found to share many morphological and life-history characters with the globular-gall morph of P. populi-transversus, including the globular gall shape and the spring sexuals noted by Faith (1979a,c). Then, in groundwork for this study, we found that the globular morph is in actuality a distinct population of P. obesinymphae, and not a morph of P. populi-transversus. We clarify this relationship here to coordinate our study with the existing literature on P. populi-transversus. Hereafter, to limit further confusion, we will recognize the species that forms the elongate gall alone as P. populi-transversus. The species that forms the globular gall is P. obesinymphae, and is distributed across the United States on two hosts: the eastern cottonwood P. deltoides (on which it is sympatric with P. populi-transversus) and the Fremont cottonwood (on which P. populi-transversus is absent).

This analysis is necessary because of the complex distribution of P. obesinymphae: as we have described, the species consists of two lineages, one sympatric with P. populi-transversus on P. deltoides, and one restricted to southwestern North America on Populus fremontii (Aoki and Moran 1994; Abbot and Moran 2002).

In early spring, a first burst of growth occurs from embryonic leaves that have overwintered in the buds. These ‘‘early’’ leaves are followed by a ‘‘late’’ flush of summer leaves, most of which are newly initiated on the growing shoots. Pemphigus populi-transversus foundresses, having overwintered as eggs, form galls on the early, spring leaves. Pemphigus obesinymphae, by contrast, returns to P. deltoides after the first flush of spring leaves, and forms galls on the late flush of summer leaves. Having appeared later on trees than P. populi-transversus, it also remains longer, well into the fall after P. populi-transversus has departed for its secondary hosts (Bird et al. 1979; Fig. 1).

As expected, there was no evidence that P. populi-transversus forms galls on Fremont cottonwoods, despite a westerly distribution on P. deltoides that overlaps with the eastern boundary of P. fremontii’s range (Fig. 2). Overall, the distribution of P. populi-transversus is in complete accord with the surveys of Sokal and associates.

- Patrick Abbot, James Withgott: (2004) Phylogenetic and molecular evidence for allochronic speciation in gall‐forming aphids (Pemphigus)^©