Journal of Nematology 35(2):133–141. 2003. © The Society of Nematologists 2003. Evolution of Parasitism in Insect-transmitted Plant Nematodes1 R. M. Giblin-Davis,2 K. A. Davies,3 K. Morris,4 and W. K. Thomas4 Abstract: Nematode-insect associations have evolved many times in the phylum Nematoda, but these lineages involve plant parasitism only in the Secernentean orders Aphelenchida and Tylenchida. In the Aphelenchida (Aphelenchoidoidea), Bursaphel- enchus xylophilus (Pine wood nematode), B. cocophilus (Red ring or Coconut palm nematode) (Parasitaphelenchidae), and the many potential host-specific species of Schistonchus (fig nematodes) (Aphelenchoididae) nematode-insect interactions probably evolved independently from dauer-forming, mycophagous ancestors that were phoretically transmitted to breeding sites of their insect hosts in plants. Mycophagy probably gave rise to facultative or obligate plant-parasitism because of opportunities due to insect host switches or peculiarities in host behavior. In the Tylenchida, there is one significant radiation of insect-associated plant parasites involving Fergusobia nematodes (Fergusobiinae: Neotylenchidae) and Fergusonina (Fergusoninidae) flies as mutualists that gall myrtaceous plant buds or leaves. These dicyclic nematodes have different phases that are parasitic in either the insect or the plant hosts. The evolutionary origin of this association is unclear. Key words: Aphelenchida, Bursaphelenchus cocophilus, Bursaphelenchus xylophilus, evolution, Fergusobia spp., plant-parasitism, Schis- tonchus spp., tritrophic associations, Tylenchida. Nematodes have independently evolved many com- families in the order Aphelenchida: the Aphelenchoi- mensal (phoretic) and parasitic associations with mem- dea, which comprises two monogeneric families of my- bers of the class Insecta (Giblin, 1987; Poinar, 1975). cophagous nematodes, and the Aphelenchoidoidea, The greatest diversity of insect-nematode associations which consists of six families of mycophagous, plant- occurs in the class Secernentea with known radiations parasitic, predatory, insect-phoretic, or parasitic nema- in the orders Rhabditida, Diplogasterida, Tylenchida, todes (i.e., Aphelenchoididae, Seinuridae, Ektaphelen- Aphelenchida, Oxyurida, Spirurida, Strongylida, chidae, Acugutturidae, Parasitaphelenchidae, and En- Rhigonematida, and Ascarida (Poinar, 1975). In con- taphelenchidae). Insect- and plant-associated members trast, the Mermithida is the only known order of insect- of the Bursaphelenchinae (Parasitaphelenchidae) in- associated nematodes that occurs in the class Adeno- clude the Pine wood nematode Bursaphelenchus xylophi- phorea. According to a recent investigation establish- lus and the Red ring nematode B. cocophilus (Giblin- ing a molecular evolutionary framework for the phylum Davis, 1993). The only other known genus in the Aph- Nematoda using small subunit (SSU) rDNA sequences, elenchoidoidea that is associated with both insects and plant-parasitism evolved at least twice in each class (i.e., plants is the potentially large radiation of host-specific Secernentea and Adenophorea) (Blaxter et al., 1998). species of Schistonchus (Aphelenchoididae) (Giblin- Yet, only a very small subset of nematode lineages is Davis, 1993; Giblin-Davis et al., 1995). known to be involved in tritrophic associations that use The pre-adaptations associated with phoretic trans- both insects and plants as hosts, and these exclusively port of Aphelenchoidoid nematodes by insects appear occur in the Secernentean orders Aphelenchida and to be critical for subsequent evolution of insect- Tylenchida (Giblin-Davis, 1993; Giblin-Davis et al., associated plant-parasitism (Giblin-Davis, 1996). These 2002). The purpose of this paper is to introduce the pre-adaptations include developmental synchrony with reader to these interesting tritrophic associations and the insect transport host in the production of a dis- speculate on how they evolved. persal stage juvenile (dauer juvenile) or adult female to Aphelenchoidoid nematode associations with insects and survive environmental stresses (e.g., starvation and (or) plants: According to Hunt (1993), there are two sub- desiccation) before, during, and after transport. Some of these stresses also could have selected for internal Received for publication 9 July 2002. phoresy, as in Schistonchus, which may have been a pre- This research was supported in part by U.S.D.A. Special Grant in Tropical cursor to the evolution of insect parasitism. and Subtropical Agriculture CRSR-99-34135-8478. This manuscript is Florida Agricultural Experiment Stations Journal Series R-08491. Insect-associated plant-parasitism in Aphelenchoi- 1 Paper delivered in a symposium on Evolution of Parasitism in Nematoda at doid nematodes probably evolved independently from the 40th Annual Meeting of The Society of Nematologists, 24–29 August 2001, Salt Lake City, UT. dauer-forming, mycophagous ancestors that were pho- 2 Professor University of Florida-IFAS, Fort Lauderdale Research and Educa- retically transmitted to breeding sites of their insect tion Center, 3205 College Ave., Davie, FL 33314-7799. 3 Visiting Research Fellow, current address Department of Applied and Mo- hosts in plants. Mycophagy probably gave rise to facul- lecular Ecology, Waite Campus, The University of Adelaide, PMB 1 Glen Os- tative or obligate plant-parasitism because of opportu- mond, South Australia, 5064. 4 Supervisor of Laboratory and Professor, respectively, Hubbard Center for nities due to insect host switches (e.g., Schistonchus), Genome Studies, University of New Hampshire, 4th Floor, Environmental peculiarities in host behavior such as maturation feed- Technology Bldg., 35 Colovos Road, Durham, NH 03824. The authors thank Helen Braasch for providing samples of Bursaphelenchus ing of the insect host on healthy plant hosts (e.g., B. xylophilus and B. fraudulentus for sequencing and Bill Howard and Gary Taylor xylophilus), or attraction of insect hosts to wounds on for review of the manuscript. E-mail: giblin@ufl.edu healthy plant hosts (e.g., B. cocophilus). The life histo- This paper was edited by B. C. Hyman. ries of B. xylophilus, B. cocophilus, and Schistonchus are 133 134 Journal of Nematology, Volume 35, No. 2, June 2003 reviewed here to further investigate the evolution of mycophagous life cycle involved in secondary transmis- insect-associated plant-parasitism in the Aphelenchoi- sion (Giblin-Davis, 1993). As conifers wilt and die, ei- doidea. ther from pine wilt disease or other causes, they be- Pine wood nematode, Bursaphelenchus xylophilus: During come suitable breeding hosts for Monochamus, and this primary transmission of the plant-parasitic phase, dauer allows for the ancestral secondary transmission pathway juveniles (JIV stage) of the Pine wood nematode B. (Fig. 1). Dauer juveniles transmigrate out of the tra- xylophilus, which are phoretically associated in the tra- cheal system of their beetle host during oviposition into cheae of their longhorn beetle hosts (usually Monocha- these dying or dead conifer hosts where they feed, re- mus spp.) (Cerambycidae: Coleoptera), are transmitted produce, and greatly increase their population densi- to young twigs of a susceptible healthy conifer host ties on fungi. The biology of the Pine wood nematode (usually Pinus spp.) (Fig. 1). This “maturation feeding” is synchronized with the development of the progeny of by a newly emerged female beetle is required for repro- the longhorn beetle, with pre-dauer juveniles (JIII) de- ductive maturity. At this point, the dauer juveniles dis- veloping as conditions in the host conifer become less embark from their host, perhaps in response to semio- suitable. This is usually correlated with the develop- chemicals released by the beetle feeding, and enter the ment of late instar larvae or pupation of its Monochamus wound to infect, feed, and develop on pine parenchy- host and results in large aggregations of pre-dauer ju- mal cells. In susceptible pines under the right condi- veniles around the pupal chambers where they overwin- tions this can lead to classical pine wilt disease that ter with the beetle. A large proportion of the pre-dauer results in the death of the tree within a year of infection juveniles of B. xylophilus molt to dauer juveniles at about (Fig. 1) (Giblin-Davis, 1993). the time of adult beetle emergence and are attracted to The maturation feeding requirement of the Monocha- CO2 produced by newly eclosed adults. These dauer mus host probably allowed for the evolution of faculta- juveniles migrate into the metathoracic spiracles where tive plant-parasitism in B. xylophilus from the ancestral most of the CO2 is emitted during beetle respiration. Fig. 1. Generalized representation of the association between the pine wood nematode, Bursaphelenchus xylophilus, its Monochamus vector, and a susceptible conifer host. Evolution of Insect-Transmitted Plant Nematodes: Giblin-Davis et al. 135 The beetle host emerges from the long-dead conifer ring in stem cross section, which can lead to a lethal wilt host and starts the life cycle over again, sometimes with in 3 to 7-year-old coconut palms within 2 to 4 months as many as 289,000 dauer juveniles (Linit, 1988). (Fig. 2) (Giblin-Davis, 1993). Dauer juveniles accumu- This association probably functions quite well with- late in the red ring tissue (up to 11,000 per gram of out the plant-parasitic phase. In fact, the Bursaphelen- tissue) and associate with larval progeny of palm weevils chus species most closely