The origin of workerless parasites in Leptothorax (s. str.) (Hymenoptera: Formicidae).
Psyche 102(3-4):195-214, 1995.
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THE ORIGIN OF WORKERLESS PARASITES IN
LEPTOTHORAX (SSTR.) (HYMENOPTERA: FORMICIDAE) Theodor-Boveri-Institut (Biozentrum der Universitat), LS Verhaltensphysiologie und Soziobiologie, Am Hubland, D-97074 Wurzburg, F.R.G.
The evolutionary origin of workerless parasitic ants parasitizing colonies of Leptothorax (s.str.1 is investigated using data on mor- phology, chromosome number, and allozyme phenotype of both social parasites and their hosts. Of the three previously proposed pathways, the evolution of workerless parasites from guest ants or slave-makers is unlikely, at least according to a phenogram obtained by UPGMA clustering of Nei's similarities based on seven enzymes. Intraspecific evolution of the workerless parasites Doronomyrmex goesswaldi, D. kutteri, and D. pacis from their common host, Leptothorax acervorum cannot be excluded with the present data. The workerless parasite L. paraxenus, however, clearly differs from its host, L. cf. canadensis, in morphology and biochemistry, and most probably did not evolve from the latter species. It is proposed to synonymize Doronomyrmex under Lep- tothorax (s. str.).
Eusocial insects by definition are characterized by a division of labor between non-reproductive workers and reproductive queens. Nevertheless, in a small minority of ant, bee, and wasp species, the worker caste has been secondarily lost. Instead of founding their own colonies solitarily, the queens of these workerless social para- sites invade the nests of other, often closely related host species and exploit the present worker force to rear their own young. In 'present address: Zool. Inst. I, Univ. Erlangen-Numberg, Staudtstrasse 5, D-91058 Erlangen, Gemany
Manuscript received 19 Junuuiy 1996.
196 Psyche [VOI. 102
ants, the queens of some parasite species kill all host queens ("murder parasites," Faber, 19691, but in other species parasite and host queen live and reproduce together (inquilines in the strict sense, e.g., Bourke and Franks, 1991). The evolution of social par- asites and in particular of socially parasitic ants has been exten- sively discussed starting with Darwin (1859). Three main routes leading to workerless parasitism have been proposed: workerless parasites might evolve a) directly from the species or species group serving them as host (Emery, 1909; Wasmann, 1909; Kutter, 1969; Buschinger, 1990; Bourke and Franks, 1991); b) from other para- sites, such as temporary parasites, slave-makers, or guest-ants (Wasmann, 1908, 1909; Emery, 1909; Wilson, 1971); or c) from non-parasitic ancestors other than the host species (West-Eberhard, 1990; Bourke and Franks, 199 1).
The myrmicine tribe Formicoxenini (formerly Leptothoracini, Bolton, 1994) is extraordinarily rich in social parasites and thus provides an ideal system to investigate the evolutionary pathways to workerless parasitism (Buschinger, 1986, 1989, 1990). Lep- tothorax (s-str.) (i.e, L. acervorum, L. muscorum, L. cf. canadensis and several other non-parasitic taxa), Formicoxenus, Harpagox- enus, and the palaearctic Doronomyrmex appear to be especially closely related (Buschinger, 198 1, 1987) and have been grouped in a distinct subtribe within the Formicoxenini (Loiselle, Francoeur and Buschinger, 1990). They nevertheless exhibit remarkably dif- ferent life histories. Formicoxenus are guest-ants, which live in the nests of Formica, Myrmica, or Manica (Francoeur et al., 1985). Formicoxenus workers beg food from their hosts but rear their own brood in separate chambers close to the host. The host colonies remain intact and continue to produce sexual brood (e.g., Wheeler, 1910). Harpagoxenus are slave-makers, whose queens after invad- ing a Leptothorax (s-str.) host colony kill or expel all adult resi- dents. Leptothorax workers which eclose from the conquered brood serve as "slaves" and take care of the slave-maker queen's larvae. Harpagoxenus workers eventually pillage brood from neighboring Leptothorax nests, which after eclosion serve as addi- tional slaves. Doronomyrmex kutteri and D. pacis are workerless parasites which tolerate the Leptothorax host queens, though they probably decrease host reproductive success by feeding on their eggs (Kutter, 1969; Franks et al., 1990). D. goesswaldi, L. para- xenus, and L. wilsoni are workerless parasites which kill the host
19951 Heinze 197
queen, but not the adult host workers (Buschinger and Klump, 1988; Heinze, 1989; Heinze and Alloway7 1991). Several attempts have been made to deduce the evolutionary ori- gin of formicoxenine murder parasites and inquilines from data on morphology, karyotype, and enzyme phenotype (Buschinger, 198 1, 1990; Heinze? 199 1). Reviewing these previously published results and providing additional unpublished data, here I critically exam- ine the hypotheses on the evolution of workerless parasites in this group and provide evidence that routes a and c are the most likely pathways leading to workerlessness.
Colonies of parasitic and non-parasitic Formicoxenini (Table 1) were collected during the last 10 years in various parts of North America, Europe, and Turkey. Ants used in this study are from the following sites. Parasites: L. paraxenus: Bic (Co. de Rimouski, Quebec), Milton (Halton Co., Ontario); L. wilsoni: Mt. Monadnock (Cheshire Co., New Hampshire), Escoumins (Co. de Saguenay, Quebec)? Jasper Nat. Park (Alberta; Buschinger and Schumann, 1994); Doronomyrmex goesswaldi: La Villette (Dept. Hautes- Alpes, France); D. kutteri: Leinburg (Bavaria? Germany); D. pacis: La Villette (Dept. Hautes-Alpes, France); Harpagoxenus sublaevis: Rudolstadt (Thuringia, Germany); Formicoxenus quebecensis: Waswanipi (Co. de Abitibi,
Mt. du Lac des Cygnes (Co.
de Charlevoix-Est, Quebec)? Jasper Nat. Park (Alberta; Buschinger, Schumann and Heinze, 1994). Non-parasitic species: L. sp. A: Tadoussac (Co. de Saguenay? Quebec); L. acervorum: Grossos- theim (Bavaria, Germany), Leinburg (Bavaria, Germany), Ilgaz Dagi Ge~idi (Cankiri? Turkey); L. cf. canadensis: Bic (Co. de Rimouski? Quebec)? Tadoussac (Co. de Saguenay, Quebec), Mount Monadnock (Cheshire Co., New Hampshire); L. gredleri: Sommer- hausen (Bavaria? Germany); L. muscorum: Leinburg (Bavaria, Ger- many)? Ilgaz Dagi Ge~idi (Cankiri? Turkey); L. ''muscorum~' C: Maligne Canyon (Alberta); L. retractus: St. Simeon (Co. de Charlevoix-Est); L. sphagnicolus: L'Ascension (Co. de Chicoutimi, Quebec); D. pocahontas: Maligne Canyon (Alberta). Details on the collecting procedure, laboratory rearing, and the life histories and collecting sites are published elsewhere (Heinze, 1989, 1993; Heinze and Ortius, 1991; Heinze, Trunzer, Lechner and Ortius, 1995).
198 Psyche [vo~. 102
Table 1. Synopsis of guest-ants, slave-makers, murder parasites, and queen-toler- ant inquilines thought to be related to the ant subgenus Leptothorax (sstr,). Geographical
Species TY ~e Host species range References Doronomyrmex murder
D. kutteri inquiline
D. pacis inquiline
L. faberi murder
L. paraxenus murder
L. wilsoni murder
L. acervorum Alps
L. acervorum Alps, S. Sweden,
L. acervorum Alps
L. cf. canadensis Maligne Lake,
L. cf. canadensis Ontario, Qukbec
L. cf. canadensis, Qukbec, New
Alloway, 199 1
parasite L. sp. A
Harpagoxenus slave-maker L. acervorum,
sublaevis and L. canadensis,
H. canadensis L. gredleri,
L. sp. A
Formicoxenus guest-ant Myrmica spp.,
SPP. Formica spp.,
Brunswick, New Heinze et al.
Hampshire, Rocky 1995,
Mountains Buschinger and
coniferous forests Buschinger,
197 1 ; Heinze,
North America Stuart,
and Eurasia Alloway, and
holarctic Francoeur et
L. cf. canadensis, the most common Leptothorax (s-str.) in New England, Qukbec and the Canadian Maritime Provinces is also referred to as ''large black L. 'muscorum~~' (e.g., Francoeur, 1986; Loiselle et al., 1990) or Leptothorax sp. B (e.g., Heinze and Buschinger, 1987, 1988; Heinze, 1989). However, the original description of L. canadensis (Provancher, 1887) fits quite nicely to this taxon. At present it is not known how far west L. cf. canaden- sis ranges, but morphologically, karyologically, and biochemically more or less similar ants (referred to as Leptothorax ''muscorum~~ D and E, Heinze, 1989) occur throughout the Rocky Mountains and the Coast Mountains in western Canada and the western USA. Leptothorax sp. A is a widespread species in open coniferous forests and on partly shaded rocky patches in New England,
19951 Heinze 199
Qukbec, Ontario, and New Brunswick; a morphologically similar species, though with a different chromosome number, Leptothorax "muscorum~~ C, perhaps identical to the variety L. muscorum septentt-ionah (Wheeler, 19 171, is found in the Canadian Rocky Mountains.
Chromosomes were prepared from unpigmented male Leptotho- rax pupae following a procedure by Imai, Crozier, and Taylor 1977; (see also Loiselle, et al, 1990). For electrophoresis in 12.5 cm long 7.5% polyacrylamide gels, adults or pupae were crushed individually in 40pl of PAGE-homogenization buffer (0.lM TrislHC1 pH 8.0, 1mM EDTA, 0.05mM NADP, 2mM p-Mercap- toethanole, 10% glycerine, 0.01 % bromothymol blue), of which 5 to lop1 were applied to the gel (gel buffer: 0-125M TrisfHCl pH 8.0; tray buffer 0.16M glycine, 0.025 M Tris, pH 8.3). Proteins were separated at 10å¡ with 10mA per gel for 1.5 hours. For detection of IDH, 0.25M TrislHCl pH 9.6 was used both in the gel and as tray buffer. For electrophoresis on cellulose acetate plates (Titan 111, Helena Laboratories, Beaumont, Texas), whole ants were crushed in 5-8pl tray buffer (with 0.01% bromothymol blue and amaranth as tracking dyes) and applied to the surface of the pre-soaked gel (tray buffer: 0.1M TrisfO. 1M MaleatlO.0lM EDTA, pH 7.4, 1 : 10) using the Helena "Super Z" applicator. Gels were run for 30 min. at 200 V at 5OC (see also Heinze, 1991). Of 14 to 20 enzymes screened in Leptothorax acervorum, L. cf. canadensis, and L. sp. A, 7 which could reliably be stained and showed a rea- sonable amount of inter- or intraspecific variation, were chosen for a more detailed analysis of both host species and parasites. In most non-parasitic species, at least 20 workers from 10 different colonies were stained for each enzyme; sample sizes are typically much larger in GPI and PGM. Fewer workers were available from L. sphagnicolus, L. retractus, and the parasitic taxa. In species with limited material, where no allozyme differences were found between populations (L. paraxenus, L. wilsoni, I? quebecensis), data from different populations were pooled for the analysis. Not all of those enzymes found to be invariable among the non-para- sitic species were stained in the workerless parasites. Nei's indices were calculated from allele frequencies and clustered (UPGMA) using the computer program NTSYS (Rohlf, 1990); in addition, a neighbor-joining tree (Saitou and Nei, 1987) was calculated. Stability of clusters in the UPGMA phenogram was tested by
200 Psyche [vo~. 102
jackknifing over taxa (Lanyon, 1985) and by calculating a cophe- netic regression coefficient.
Voucher specimens, wherever available, of the studied species are deposited in the MCZ, Cambridge, Mass., (USA). Morphology
The current taxonomic confusion concerning the nearctic repre- sentatives of Leptothorax (s.str.) (e.g., Creighton, 1950; Brown, 1955; Francoeur et al., 1985; Heinze, 1989) makes a thorough morphological comparison between non-parasitic Leptothorax (s.str.) and the associated parasitic taxa difficult. Only few charac- ters appear to be stable enough to serve for species distinction (Table 2). The most reliable and most frequently cited characters are the suberect hairs on tibia and scapes in L. acervorum, L. sphagnicolus, Doronomyrmex, and Harpagoxenus, and the indented clypeus in L. gredleri, L. retractus, L. paraxenus, and to a lesser extent also Leptothorax sp. A (Table 2). Both characters, however, may vary between populations: L. acervorum from Alaska, for example, are considerably less hairy than central Euro- pean specimens (Heinze and Ortius, 1991). Other characters which serve to distinguish parasitic genera from Leptothorax (s. str.) are probably mostly adaptations to the parasites' specialized way of life and cannot be used to investigate phylogenetic relationships. Harpagoxenus, e.g., is easily recognized by strong, toothless mandibles and an extraordinarily large head with antenna1 scrobes, all obvious adaptations to slave-making. Similarly, the morpholog- ical features separating Doronomyrmex from Leptothorax are thought to be adaptations to the parasitic life, and some authors question whether Doronomyrmex should be kept as a distinct genus (Brown, 1973; Bolton, 1982).
In a detailed morphological revision, Francoeur et al. (1985) provided clear evidence for the separation of the guest-ant genus Formicoxenus from non-parasitic Leptothorax (s.str.). Female Formicoxenus are more slender than Leptothorax and the scape of Formicoxenus males is longer and more cylindrical than in Lep- tothorax males. Furthermore, whereas the majority of Formicox- enini, including non-parasitic Leptothorax and Harpagoxenus,
19951 Heinze 201
Table 2. Morphological characteristics and chromosome numbers of Leptothorax (s.str.) and their parasites, based on Bolton (1982), Francoeur et al. (1985), Loiselle et al. (1990), and own observations.
Eyes Erect Enlarged
with hairs on Dufour's Clypeus Palp
hairs legs gland indented formula Chromosomes L. acervorum
L. cf. canadensis
L. sp. A
4,3 and 5,3
4,3 or 5,3
have a standard palp formula of 5 maxillary palps and 3 labial palps, the number may be reduced to 4, 3 in Formicoxenus (Bolton, 1982). However, as shown by Francoeur et al. (1985), there is intergeneric and even intraspecific variation in palp formula in Formicoxenus. The palp formula is similarly reduced to 4, 3 in Leptothorax wilsoni (two queens from Jasper N.P., Alberta, and one individual from Escoumins, Quebec-the palp formula given in Heinze, 1989 for the type material needs to be confirmed by re- examination of material from the type locality) and Doronomyrmex pacis (two individuals from Jenner, Germany; according to Kutter, 1950 males of D. pads from Switzerland have a palp formula of 5, 4). Palp formula is therefore probably not very informative on genus level (Table 2).
202 Psyche [vo~. 102
Formicoxenus have scattered hairs on their compound eyes, whereas the eyes are thought to be hairless in Leptothorax (Fran- coeur et al., 1985). Hairy eyes, however, are found in L. wilsoni (Heinze, 1989; Table 2). Despite the superficial similarity between L. wilsoni and Formicoxenus in these two characters, L. wilsoni is nevertheless morphologically closer to Leptothorax in others, such as the scape length in males. L. wilsoni shares reduced mandibular dentition with Epimyrma, a slave-making satellite genus of Lep- tothorax (Myrafant), but clearly differs in palp formula and the shapes of petiole and postpetiole.
Most queens of workerless parasites in Formicoxenini, and per- haps of workerless parasitic ants in general, are extraordinarily small compared to queens of the host species (Douwes, 1990; Nonacs and Tobin, 1992). Queens of L. paraxenus are a notable exception, in that they are of similar size as the host queens. As small size is thought to be adaptive in parasites-parasite queens do not need much resources for colony founding and thus, a larger number of less well equipped, small queens can be produced from the same amount of energy available to the colony (Douwes, 1990)-the condition in L. paraxenus might reflect a recent origin from a non-parasitic ancestor.
Parasite queens are typically characterized by a broadened post- petiole, a strong ventral petiolar spine, and a larger Dufour's gland compared to their hosts. These features, however, are not restricted to social parasites but may be found to a varying degree also in the queens of free-living species such as Leptothorax sp. A and L. gredleri.
Chromosome Number and Allozyme Phenotype The significance of chromosome number as a taxonomic charac- ter is very poorly understood. Nevertheless, various studies have used chromosome analyses to clarify the taxonomy of Formicox- enini (Fischer, 1987; Heinze and Buschinger, 1989; Loiselle et al., 1990; Buschinger and Fischer, 199 1). Chromosome numbers in Leptothorax (s.str.) and associated genera range between 11 and appr. 28. Palaearctic Doronomyrmex have much higher chromo- some numbers than their common host, L. acervorum. In contrast, the nearctic workerless parasites L. faberi (Buschinger, 1982) and L. paraxenus both have 15 chromosomes (L. paraxenus: 30 metaphase plates from 4 male pupae from Milton, Ontario),
19951 Heinze 203
whereas their host L. cf. canadensis has 17 or 18 (Heinze and Buschinger, 1989; Loiselle et al., 1990). UPGMA clustering of Nei's indices calculated from seven enzyme systems which are of diagnostic value in the studied species results in the phenogram shown in Fig. 1. Original data and Nei's indices are given in Tables 3 and 4. Goodness of fit of the cluster analysis to the data was tested by comparing a matrix of cophenetic values, calculated from the tree matrix, with the origi- nal similarity matrix. The resulting cophenetic correlation of r = 0.809 suggests a rather mediocre fit (Mantel t-test, t = 6.53, p = 1.000). Nevertheless, the overall branching pattern is remarkably stable. Two fundamental dichotomies are found in all 21 Fig. 1. Phenogram obtained by UPGMA clustering of Nei's indices from electro- morph frequencies of several species of Leptothorax (s.str.), Doronomyrmex, Formi- coxenus quebecensis, and Harpagoxenus sublaevis. For abbreviations see Table 3.
204 Psyche [vol. 102
pseudoreplicates generated from the original data set by jackknif- ing over taxa. Firstly, Formicoxenus, represented in the elec- trophoretical study by F, quebecensis, is least similar to the other taxa, and secondly, a cluster consisting of Harpugoxenus sublaeuis, L. retractus, L. cf. canadensis, and L. wilsoni (cluster A), stands in sharp constrast to the remaining taxa (L. acervorum, L. sphagnico- lus, L. muscorum, L. gredleri, L. paraxenus and Doronomyrmex, cluster B). L, acervorum, L. sphugnicolus and the two workerless parasites, D. kutteri and D. goesswaldi, also form a stable group (supported in all pseudorepHcates), which is probably reflected in morphological similarities among these species, such as the pres- ence of erect hairs on the scapes and legs. Though Harpagoxenus and D. pads likewise share this character, at least Harpagoxenus appears biochemically quite dissimilar from the L. acervorum group. The branching pattern in the lower half of cluster B (L. muscorum, L. gredleri, L. sp. A, L. paraxenus, D. pacts, and D. pocahontas} was present in only 19 of 21 replicates and thus is less well supported by the data. It therefore would be premature to con- clude that D. pads is not very close to its workeriess congeners Table 3. Frequency of electromorphs of 7 enzymes in rite ant genus LeptMhorax kstr.) and associated parasitic taxa. The abbreviations stand for the followinc .,
species: Dgoe: 0. goesswaldi (9 virgin queens from 3 colonies); Dku: D. kutieri (8 virgin queens from 2 colonies); Dpac: D. pacis (5 virgin queens from 3 colonies); Dpoc: D. pocahontax (10 workers from 3 colonies); Fqu: Formicoxenus quebecewis (12 workers from 4 colonies from different populations); Hx: Harpapxenus sublae- vis (26 workers from 8 colonies); LA; L. sp. A (209 workers from 17 colonies; LaG, LaR, LaT: L. acervorum from Gropostheirn (Germany, 196 workers from 89 colonies), Reichswald (Germany, between 64 and 1420 workers from 10 to 140 colonies), and Ilgaz Dagi Gyidi (Turkey, 49 workers from 7 colonies); LC: Lep- tothorax C (10 workers from 4 colonies); LcB, kM, LcT: L. cf. canadensis from Bic (Quebec, ! 13 workers from 13 colonies), Mt. Monadnock (New Hampshire, 131 workers from 17 colonies) and Tadoussac (Qud~w. 66 workers from 10 colonies); Lgr: L. gredleri (between 20 and 299 workers from 29 colonies); hu, LmuT, L. muscorum from Reichswald (between 20 and 569 workers front 76 colonies) and Ilgaz Dagi Ge~idi (20 workers from 5 colonies); Lpm L. paraxenus (10 virgin queens from 4 colonies}; Let: L retractus (10 workers from 2 colonies); Lsp: L. sphagnicolus (10 workers from 2 colonies); h i : L witsod (8 virgin queens from 4 colonies).
The enzymes are Glucose-6-phosphate isomerase (GPI, referred to as PGI in Hei me, 1989, 199 1). Phosphoglucomutase (EM), 6-Phosphogluconate dehydmge- nase (PGD), Malate dehydrogenase (MDH), Ismitrate dehydrogenase (IDH), Matic enzyme (ME), and Lactate dehydrogenase (LDH). All species share the same elm- tromorph of tetrazolium oxidase. v, s, rn, n, f, and x denote different migration velocity during electrophoresis; blanks in the table indicate missing data.
Table 4. Nei's indices calculated from electromorph frequencies in Table 3. - - - - - - pp
Dgoe Dku Dpac Dpoc Fqu HxLA LaG LaR L a T L C LcB LcM LcT Lgr Lmu LmuT Lpar Lret Lsp Lwi Dgoe 0.000
Dku 0.044 0.000
Dpac 0.221 0.288 0.000
Dpoc 0.381 0.582 0.288 0.000
Fqu 1.055 0.847 0.693 1.925 0.000
Hx 0.650 0.656 0.693 0.589 0.794 0.000
LA 0.636 0.836 0.405 0.215 1.108 0.353 0.000 LUG 0.018 0.030 0.223 0.407 1.059 0.437 0.531 0.000 LaR 0.032 0.048 0.246 0.399 1.107 0.418 0.517 0.008 0.000 La! 0.025 0.076 0.260 0.368 1.201 0.501 0.477 0.025 0.013 0.000 LC 0.362 0.560 0.288 0.004 1.946 0.656 0.213 0.394 0.398 0.352 LcB 0.826 0.834 1.370 1.166 0.947 0.323 0.610 0.595 0.498 0.523 1.270 0.000 LcM 0.823 0.830 1.349 1.174 0.929 0.317 0.608 0.590 0.496 0.525 1.279 0.001 0.000 LcT 0.827 0.798 1.373 1.189 0.917 0.318 0.619 0.582 0.490 0.519 1.294 0.001 0.001 0.000 Lgr 0.357 0.449 0.279 0.350 1.208 0.316 0.568 0.267 0.262 0.288 0.370 0.730 0.727 0.733 0.000 Lmu 0.127 0.294 0.272 0.176 1.348 0.342 0.369 0.127 0.134 0.125 0.167 0.705 0.701 0.713 0.153 0.000 LrnuT 0.139 0.154 0.288 0.350 0.981 0.342 0.553 0.109 0.157 0.196 0.336 0.940 0.925 0.912 0.21 1 0.1 11 0.000 Lpar 0.362 0.560 0.288 0.004 2.079 0.543 0. I83 0.331 0.332 0.299 0.000 0.987 0.991 1.002 0.314 0.144 0.288 0.000 Lret 0.468 0.313 0.693 1.145 0.487 0.218 0.716 0.365 0.339 0.443 1.229 0.337 0.331 0.313 0.689 0.775 0.536 1.229 0.000 Lsp 0.044 0.154 0.288 0.350 1.386 0.543 0.456 0.071 0.043 0.013 0.336 0.484 0.491 0.488 0.308 0.131 0.288 0.288 0.536 0.000 Lwi 1.055 1.253 1.386 1.232 0.981 0.438 0.768 0.843 0.872 0.939 1.253 0.655 0.659 0.661 0.669 0.677 0.693 0.981 0.824 0.981 0.000
19951 Heinze 207
and L. acervorum, as suggested by the phenogram in Fig. 1. It has recently been shown that D. pocahontas is not a workerless para- site and thus does probably not belong to the workerless Dorono- myrmex. The shiny, long-haired queens, morphologically similar to D. pacis but strikingly different from their nestmate workers, are probably a special morph of a species whose queens and workers are typically short-haired and dull (Buschinger and Heinze, 1993). In a previous study using cellulose acetate electrophoresis, "shiny" queens and workers of D. pocahontas from three colonies from the type locality differed from other studied species in having a very slowly migrating electromorph in the enzyme 6-Phosphogluconate dehydrogenase (PGD, Heinze 1989). According to the results of the present study with polyacrylamide gels, workers from three colonies with "dull" queens from the same collecting site, how- ever, predominantly had the slowly migrating PGD electromorph found commonly also in other taxa in cluster B (Table 3). In one colony, workers were heterozygous for the slow and the fast migrating electromorph. More data on this ant and morphologi- cally similar taxa from the Rocky Mountains are needed to eluci- date the status of D. pocahontas.
L. paraxenus differs from its host, L. cf. canadensis in several enzymes (Table 3) and is attached to cluster B. L. wilsoni is closely associated neither with one of its two hosts, L. cf. canadensis and L. sp. A, nor with Formicoxenus, but is more similar to species in cluster A than to cluster B species.
In a neighbor joining tree, which, however, showed poorer fit to the original data (r = 0.691, t = 5.174, p = 1.000), F. quebecensis is attached to L. retractus and L. cf. canadensis, whereas L. wilsoni and Harpagoxenus are closer to the taxa of cluster B. Morphology, allozyme phenotype, and chromosome number were compared to analyze the evolution of workerless social para- sites in the ant tribe Formicoxenini. Though the data are as yet not sufficient for a thorough cladistic analysis, similarities in morpho- logical and biochemical characters which I consider to be neutral in natural selection support the results of previous more qualitative studies (Buschinger, 198 1, 1990; Heinze, 199 1).
208 Psyche [vo~. 102
The slave-making and workerless parasites studied here appear to be phylogenetically close to their hosts. Though the workerless L. wilsoni and the guest-ant genus Formicoxenus differ in several enzyme systems and morphological features from other taxa inves- tigated in this study, they clearly belong to the Formicoxenini and within this tribe are probably closer to Leptothorax (s.str.) and its parasites than to Leptothorax (Myrafant) or other Leptothorax sub- genera (see also Buschinger, 1981; Francoeur et al., 1985; Heinze, 1991). Harpagoxenus slave-makers and workerless Leptothorax and Doronomyrmex clearly cluster with species of Leptothorax (s.str). The data thus support what has been called "a loose version of Emery's rule." In 1909, Emery suggested for slave-makers and workerless parasites that they "all originate from closely related forms which serve them as slave or host species," based on mor- phological similarities between parasites and hosts. Ward (1989) proposed to distinguish between a loose form of Emery's rule- parasites and their hosts are close relatives (see also Wasmann, 1909)-and a strict form-for a given parasite-host pair, the sister group of the parasite lineage includes the host species (Ward, pers. comm.). The loose version appears to hold in all host-parasite pairs investigated so far (Ward, 1989; Agosti, 1994; Sanetra, Heinze and Buschinger, 1994), with exception of guest ants which explicitly were never included in this rule. A relationship as suggested by the strict version, resulting from sympatric or allopatric speciation, cannot be disproven for some formicoxenine ants. Allozyme simi- larities suggest a close relationship between at least two of the three palaearctic, workerless Doronomyrmex and their common host, L. acervorum, as previously deduced from morphological characters (e.g., the presence of suberect hairs on scapes and tibia (Buschinger, 1990)), sequence comparisons of a mitochondria1 cytochrome b gene (Baur, Sanetra, Chalwatzis, Buschinger and Zimmermann, 1995), and an internal transcribed spacer adjacent to the 5.8s rRNA gene (Baur, Sanetra, Chalwatzis, Buschinger and Zimmermann, 1996). On the other hand, L. acervorum, L. gredleri, L. muscorum, and other taxa in cluster B are very similar in almost all studied enzyme systems, and it therefore cannot be ruled out that the three Doronomyrmex originated jointly or independently from ancestors other than L. acervorum.
Morphological similarities between the Doronomyrmex species and L. acervorum might be convergent adaptations to parasitic life with a common host species instead of synapomorphies. Morpho- logical traits of parasites might serve to "camouflage" the parasite in the host colony or to increase the parasite queen's colony found- ing success. Hair length, e.g., may vary within a monophyletic group, perhaps in response to the condition in the host: the degen- erate slave-maker Epimyrma kraussei has long hairs like its host, Leptothorax recedens, but much other data suggests a common ori- gin of long- and short-haired Epimyrma (Buschinger, 1989). In any case, the result that the workerless Doronomyrmex are closer to some Leptothorax (s.str.) (cluster B) than to others (cluster A) clearly corroborates the view that the genus Doronomyrmex ought to be synonymized under Leptothorax (s.str.) (Brown, 1973; Bolton, 1982). Alternatively, Leptothorax (s.str.) would be a para- phyletic taxon. I therefore propose to synonymize Doronomyrmex under Leptothorax (s.str.).
The strict version of Emery's rule is not supported in L. para- xenus and L. wilsoni. Both parasites are clearly distinct in morphol- ogy, karyotype, and enzyme pattern from their common host, L. cf. canadensis. L. wilsoni was recently found in colonies of a second host species, Leptothorax sp. A (Heinze et al., 1995), but a closer phylogenetic relationship between them also appears unlikely based on morphology and enzyme patterns.
Buschinger provided quite firm evidence that in the formicoxe- nine genera Chalepoxenus (Buschinger et al., 1988) and Epimyrma (Buschinger, 1989) workerless parasites evolved from non-para- sitic ancestors via slave-makers, but rejected this evolutionary pathway for Doronomyrmex (Buschinger, 1990). The data pre- sented in this study indeed do not support the assumption that workerless Doronomyrmex and Leptothorax are degenerate slave- makers or have evolved from guest-ants. L. wilsoni shows some morphological and biochemical similarities to Formicoxenus (e.g., hairy eyes and a reduced palp formula), but is more similar to Lep- tothorax (s.str.) in other morpholgical features; its current system- atic position is unclear. From morphological similarities, high chromososome number, and intergeneric attractiveness of sexual pheromones, Buschinger (1990) concluded that Harpagoxenus and Doronomyrmex probably form a monophyletic group. The
19951 Heinze 211
has previously been considered by West-Eberhard (1990) and Bourke and Franks (1991) to explain the occurrence of parasite- host pairs only loosely following Emery's rule: in a species with alternative reproductive strategies, a phenotype adapted to para- sitizing conspecific colonies might have switched from its conspe- cific ancestral host to a new host species. This study was supported by the Deutsche Forschungsgemein- schaft (Heisenberg-grant He 162316-1). I thank S. Trenkle for tech- nical assistance, S. Kauffmann, B. Hulsen, S.P. Cover, T.M. Alloway and others for help in the field, and A. Buschinger and R. Schumann for material of L, wilsoni and Formicoxenus quebecen- sis from populations in western North America. P. S. Ward, Univ. of California, Davis, provided helpful comments on the manu- script.
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Note added in the proof:
Further investigations since the submission of the manuscript have led to small changes in allele frequencies, which slightly affect the branch- ing pattern in cluster B. The overall pattern, however, remained stable.
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