Phylogenetic Relationships of the Suborders of Coleoptera

Introduction

The order Coleoptera contains a richness of species which is unrivaled in the natural world. With over 350,000 described species and an estimates of extant species ranging in the several millions, beetles represent the single most successful group of organisms on earth (Caterino et al., 2002). Beetles are easily distinguished from other insects by their forewings which have been hardened and modified into protective covers (elytra) for their membranous hind wings. Other derived features of the Coleoptera include a simplified thoracic exoskeleton and reduced thoracic muscles (Maddison, 2002). The monophyly of Coleoptera is widely recognized and supported by 28 derived morphological features of adults (Beutel and Haas, 2000).

Figure 1: Phylogenetic relationships of the major groups of the Coleopteroid assemblage, placing Strepsiptera sister to the Coleoptera (Kukalova-Peck and Lawrence, 1993).

Part of the Endopterygota (Holometabola), the order is often shown as sister to the Strepsiptera (Figure 1), though this is a widely debated issue (Kukalova-Peck and Lawrence, 1993). Compared with other large insect orders, the phylogenetic relationships of the major lineages of Coleoptera are relatively unknown (Maddison, 2002). The order Coleoptera is comprised of 4 suborders: Archostemata, Myxophaga, Adephaga, and Polyphaga. The Adephaga and Polyphaga contain over 99% of all beetle species, with the vast majority being in Polyphaga. The monophyly of each of the 4 suborders is widely accepted. The enormous diversity of the order has proved to be the major obstacle to reconstruction of its major lineage phylogeny since it makes establishing comprehensive character matrices difficult or impossible (Caterino et al., 2002).

Archostemata
The suborder Archostemata contains the most primitive living beetles. Good fossil representation and a scattered world distribution indicate that it is an ancient group that was once more common throughout the world (White, 1983). Archostemata is a small suborder, containing one superfamily, Cupedoidea, and only 3 families: Cupedidae, Micromalthidae, and Ommatidae (Lawrence and Britton, 1994). Members of this suborder have thread or beadlike antennae and often have elytra which still show the remnants of wing venation (White, 1983).

Myxophaga
The other small suborder of Coleoptera is Myxophaga. This suborder also consists of one superfamily, Microsporoidea, and only 4 families: Microsporidae, Hydroscaphidae, Cyanthoceridae, and Torridincolidae (Lawrence and Britton, 1994). Myxophaga contains small, uncommon, aquatic beetles with weakly clubbed antennae. There is only one extant species of Myxophaga in North America, Hydroscapha natans (LeConte), whose larvae and adults live in the filamentous algae of streams (White, 1983).

Adephaga
Adephaga is the second largest suborder of Coleoptera. As with Archostemata and Myxophaga, it consists of just one superfamily, Caraboidea, but its 9 families contain a significantly greater diversity of species. Families within Adephaga include Carabidae, Dytiscidae, and Gyrinidae (Lawrence and Britton, 1994). Adephagan beetles are distinctive for having their first abdominal segment divided by the hind coxae and are nearly always predaceous (White, 1983).

Polyphaga

The vast majority of beetle families and species are within the suborder Polyphaga. When compared to the other three suborders, Polyphaga represents an explosion in coleopteran diversity. Polyphaga is comprised of 16 superfamilies including Scarabaeoidea, Staphylinoidea, Hydrophiloidea, and Curculionoidea. These families contain a staggering 144 families which display an enormous variety of specialization and adaptation. Polyphagan beetles do not have their first abdominal segment divided as Adephagans do. There are no typical Polyphagan characteristics since the suborder contains such a wide variety of body forms and adapted structures (White, 1983).

Figure 2: “Polyphaga + Myxophaga” hypothesis (Maddison, 2002).

Discussion

Proposed Phylogenies

The two most widely debated phylogenies proposed for the suborders of Coleoptera differ primarily in their placement of Polyphaga. The “Myxophaga+Polyphaga” phylogeny (Figure 2) was first proposed by Crowson in 1960 and has subsequently been supported by many other studies. Evidence for the relationship of Polyphaga and Myxophaga in this phylogeny is based on a shared reduction in the number of larval leg articles. Evidence for the sister relationship of Adephaga to Myxophaga+Polyphaga includes complete sclerotization of elytra, reduced number of hind wing crossveins, and folded (rather than rolled) hind wings in the three suborders (Maddison, 2002). A 2000 study by Beutel and Haas attempted to evaluate a broad spectrum of both external and internal structures in larvae and adults from all suborders. Characters evaluated included muscular features, skeletomuscular structure, wing venation, and wing articulation. All trees resulting from this study supported the monophyly of all suborders and the “Myxophaga+Polyphaga” branching pattern (Beutel and Haas, 2000).

Figure 3: “Basal Polyphaga” hypothesis (Maddison, 2002).

The alternative hypothesis to “Myxophaga+Polyphaga” was first proposed by Lawrence and Newton in 1982 and later supported by Kukalova-Peck and Lawrence in 1993. This “Basal Polyphagan” phylogeny (Figures1 and 3) places Polyphaga as sister group to the rest of Coleoptera with Adephaga being sister to Myxophaga (Maddison, 2002). Evidence for the Polyphagan relationship to the remaining suborders is based on characters of wing venation and articulation (Kukalova-Peck and Lawrence, 1993). Other evidence for “Basal Polyphaga” includes the loss of cervical sclerites in all suborders but Polyphaga (Maddison, 2002).

Figure 4: Unrooted tree based on analysis of Coleoptera alone (Caterino et al., 2002).

Molecular Phylogenies

The use of DNA in Coleoptera phylogeny reconstruction adds yet another set of characters that may help to resolve the major lineages of this diverse group. However, since the use of DNA characters in phylogeny reconstruction is in its infancy, the proper analysis and application of DNA sequence data is still being worked out. In 2002, Caterino et al. published a study of the basal relationships of Coleoptera inferred from 18s rDNA sequences. The analysis of 18s rDNA sequences for Coleoptera alone yielded an unrooted tree that supported the monophyly of each suborder (Figure 4). This tree placed Polyphaga sister to Adephaga and Archostemata sister to Myxophaga. The authors then attempted to root this tree using a wide variety of insect orders, but all resulted in a non-monophyletic Coleoptera. Realizing that the monophyly of Coleoptera is essential to a an acceptable tree, Caterino et al. used reweighed parsimony to impose it as a constraint in their analysis. However, imposing the constraint of Coleoptera monophyly only yielded trees with paraphyletic suborders. The authors then decided to impose monophyly at both the ordinal and subordinal levels which resulted in a phylogeny they considered incompatible with the “Polyphaga+Myxophaga” and “Basal Polyphaga” hypotheses. This new phylogeny placed Polyphaga sister to Adephaga with Myxophaga sister to the Polyphaga+Adephaga clade and Archostemata sister to the rest of the suborders (Figure 5). The authors themselves were skeptical of their results, noting that the smaller suborders were minimally sampled and needed to be better represented in future DNA sampling. They believed that once these sampling gaps were filled, 18s rDNA would become a more useful tool in Coleoptera phylogeny reconstruction (Caterino et al. 2002).

Figure 5: Most parsimonious tree found by reweighted parsimony enforcing the monophyly of the order Coleoptera and each of its suborders (Caterino et al., 2002).

Conclusion

The incredible species richness and diversity of this organismal linage are what simultaneously make the work of Coleoptera phylogeny reconstruction so important and problematic. The sheer variability of morphological characters among and within the four suborders makes the establishment of comprehensive character matrices difficult or impossible. For these reasons, the phylogeny of the major linegaes of the order remain relatively unclear. The two most heavily debated hypotheses for the phylogeny differ primarily in their placement of the largest suborder, Polyphaga. Most research to this date has supported the “Polyphaga+Myxophaga”, but two studies of wing characters have strongly supported the alternative “Basal Polyphaga” hypothesis (Maddison, 2002). DNA sequencing has often been proposed as a useful addition to suborder character matrices. However, phylogenies based on 18s rDNA sequences have also proven to be problematic by contradicting the well-established monophyly of the order and each of its suborders (Caterino et al., 2002). With continued research into the relationships among the four suborders of Coleoptera, including further DNA sequencing, the answer to this great puzzle may still be found. An understanding of the basal relationships the single most diverse organismal lineage will offer us insight into the adaptive radiation in insects and the evolutionary process itself.

Link to list of beetle families.

Literature Cited

Beutel, R. G. and F. Haas. 2000. Phylogenetic Relationships of the Suborders of Coleoptera (Insecta). Cladistics. 16: pp. 103-141.

Caterino, M. S., V. S. Shull, P. M. Hammond, and A. P. Vogler. 2002. Basal relationships of Coleoptera inferred from 18s rDNA sequences. Zoologica Scripta. 31: pp. 41-49.

Kukalova-Peck, J. and J. F. Lawrence. 1993. Evolution of the Hind Wing in Coleoptera. The Canadian Entomologist 125: pp. 181-258.

Lawrence, J. F. and E. B. Britton. 1994. Classification and Keys. In: Australian Beetles. Melbourne University Press, pp. 36-38.

Maddison, D. R. 2002. Tree of Life Web Project. http://tolweb.org/tree?group=Coleoptera&contgroup=Endopterygota.

White, R. E. 1983. A Field Guide to the Beetles of North America. Houghton Mifflin Company, Boston.

©2003 S. A. Stephens