Jon Chen Virtual Poster

Abstract

The genus Cyphomyrmex belongs to the tribe Attini, a group of New World ants distinguished by their ability to grow fungus. Cyphomyrmex is divided into two groups, the strigatus group and the rimosus group, differing in the states of three morphological characters: the preocular carinae, the mid-pronotal tubercles, and the number of teeth on the mandibles. This division of the genus based on morphology corresponds with a division based on an important behavioral characteristic. Some but not all members of the rimosus group grow yeast gardens (consisting of clumps of unicellular yeast cells) rather than the typical mycelial gardens (consisting of connected hyphae) grown by members of the strigatus group and by the rest of the attines. A cladistic analysis of 24 morphological characters and 22 Cyphomyrmex and one outgroup species produced a phylogeny in which the strigatus and rimosus-groups are sister clades. The yeast-growing rimosus-group species are, however, paraphyletic with respect to the yeast-growing trait, because two mycelium-cultivating sister species arise from within this group. The phylogeny is thus consistent with the hypothesis that yeast gardening is a derived trait, but indicates that there was, subsequently, a single reversal back to mycelium cultivation in the evolution of the rimosus group. Additional phylogenetic and behavioral data will be required to fully test these conclusions.

Introduction

Fungus-growing ants are classified as the tribe Attini, which belongs to the subfamily Myrmicinae of the family Formicidae, the ants. Attine ants are unique within the Formicidae because they are the only ants that cultivate fungus as a source of food. The most recognizable members of this group are the leaf-cutter ants. Members of the tribe Attini are only found in the New World and primarily in the Neotropics. Twelve of the thirteen genera within this tribe cultivate mycelial gardens (consisting of connected hyphae). The attine genus Cyphomyrmex is an exception to this rule because it contains species that cultivate yeast gardens (consisting of clumps of unicellular yeast cells). Cyphomyrmex is divided into two groups, the strigatus group and the rimosus group, differing in the states of three morphological characters: the preocular carinae, the mid-pronotal tubercles, and the number of teeth on the mandibles. This division of the genus based on morphology corresponds with a division based on an important behavioral characteristic: all members of the strigatus group cultivate mycelial gardens, while some but not all members of the rimosus group grow yeast gardens.

Current research has disproved an earlier hypothesis that this genus was the basal attine clade and has shown that yeast-growing is a derived rather than a primitive attine characteristic. Through cladistic analysis of morphological characters, this project will investigate whether yeast cultivation is a derived trait within the genus Cyphomyrmex and, if so, whether it has arisen a single or multiple times.

Methods

26 Cyphomyrmex species and a single Trachymyrmex species were used in the analysis. The specimens came from the Smithsonian Institution National Museum of Natural History and the Harvard University Museum of Comparative Zoology collections. 24 morphological characters were defined based on Kempf (1964, 1965) and Snelling and Longino (1992), and through personal inspection. Character states were determined by direct observation with a Wild Heerbrugg M5 microscope; by reference to digital images produced with a JVC KY-F70B video camera mounted on a Leica M420 microscope and attached to an IBM Intellistation M Pro computer running Auto-Montage Version 3.04 software (Synoptics Ltd.); and by reference to the taxonomic keys of Kempf (1964, 1965) and Snelling and Longino (1992). A data matrix was constructed using the computer program MacClade (Classroom Version 4.0; Maddison and Maddison, 2000). The matrix, containing the character-state assignments, appears at right. The data matrix was read into the computer program PAUP (Version 4.0b10; Swofford, 2000), which was used to conduct a cladistic analysis. The outgroup was T. zeteki. Of 23 non-binary (i.e., multistate) characters, only character 8 was ordered. A heuristic search was conducted using 100 random-taxon-addition replicates and the TBR branch-swapping algorithom. The resulting set of most-parsimonious trees were used as the starting point for a successive-approximations-weighted (SAW) analysis. Each SAW round consisted of 100 random-taxon-addition replicates utilizing TBR branch-swapping.

Results

The unweighted analysis found 2360 most-parsimonious trees (MPTs) of length 96, CI=.344, RI=.632. SAW analyses stabilized after 4 rounds and identified 5 trees, a subset of the MPTs. In the resulting trees, the strigatus and rimosus-- groups are sister clades. The yeast-growing rimosus group species are, however, paraphyletic with respect to the yeast-growing trait because two mycelium-cultivating sister species, C. costatus and C. wheeleri, arise from within this group.

Discussion

The resulting phylogeny indicates that the yeast-growing rimosus-group species are paraphyletic with respect to the yeast-growing trait, because two mycelium-cultivating sister species arise from within this group. The phylogeny is thus consistent with the hypothesis that yeast gardening is a derived trait, but the derived placement of the mycelium-cultivating C. costatus and C. wheeleri within this group indicates that there was, subsequently, a single reversal back to mycelium cultivation in the evolution of the rimosus group. Character 9, the width of the frontal lobes, appears to have a powerful influence over the arrangement of rimosus-group species. C. foxi, C. longiscapus, and C. podargus all have narrow frontal lobes that do not extend to the eye margin. They share this state with all species in the strigatus group. Since it is unknown whether C. foxi and C. podargus cultivate yeast or mycelial gardens, their phylogenetic position suggests that they cultivate the latter. Additional phylogenetic and behavioral data will be required to fully test these conclusions.

Acknowledgements

This research was supported by a grant to the NMNH Research Training Program from the National Science Foundation Research Experiences for Undergraduates program, Award Number DBI-0243512. Special thanks to Faridah Dahlan, Eugenia Okonski, and Jeffrey Sossa for all their support.

Cited References

Kempf, W.W. 1964. A revision of the Neotropical Fungus-growing ants of the genus Cyphomyrmex Mayr. Part I. Group of strigatus Mayr (Hym. Formicidae). Studia Entomologica, vol 7: 1-40

Kempf, W.W. 1965. A revision of the Neotropical Fungus-growing ants of the genus Cyphomyrmex Mayr. Part II: Group of rimosus (Spinola) (Hym. Formicidae). Studia Entomologica, vol 8: 161-400

Quintero, D., and A. Aiello (editors). 1992. Insects of Panama and Mesoamerica: selected studies. Oxford University Press, Oxford. 479-494.

Maddison D.R. and W.P. Maddison. 2000. McClade. Classroom Version 4.0. Sinauer Associates, Sunderland, Massachusetts.

Synoptics Ltd. 2003. Auto-Montage Version 3.04 (computer software). United Kingdom: Cambridge.

Swofford, D. L. 2001. PAUP*. Phylogenetic Analysis Using Parsimony (*and Other Methods). Version 4. Sinauer Associates, Sunderland, Massachusetts.

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The information presented here represents preliminary research as the result of ten-weeks of investigation in-residence at the National Museum of Natural History. This is not an official publication of the information.

As preliminary information, results and/or findings should not be cited as part of conclusive work. Please contact the authors first if you wish to utilize the information presented here.


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Research Training Program VIRTUAL POSTER SESSION 2004

Research Training Program
VIRTUAL POSTER SESSION
2004