The Gall Wasp Genus Cynips: A Study in the Origin of Species/Mutations
MUTATIONS
The most brilliant contribution to the species problem has been the outcome of transferring the search for the cause of variation into an investigation of the factors responsible for the uniformity of individuals of successive generations. Out of this development of modern genetics has come not only a localization in the reproductive cells of the mechanism by which hereditary similarities are achieved, but an assurance that the inception of new species must take place in those same genes and a considerable acquaintance with the potentialities of genes.
These laboratory studies have always led to the conclusion that changes in genes occur in sudden jumps, sometimes of small degree, sometimes of considerable size, but always as mutations which are complete as soon as they have occurred. This concept has been held in contrast with the neo-Darwinian conception of “fluctuating variations” which, by being accumulated over many generations and bent in a given direction by the force of natural selection, would gradually give rise to the characters of new species. Under this latter interpretation there may be incipient species of the sort conceived by many taxonomists in their definitions of varieties and subspecies; under the genetic interpretation the first mutant individual embodies all that the new species will contain, and is the new species as soon as it is given an opportunity to perpetuate its mutant characters thru a population of individuals.
The genetics data are conclusive as to the frequent occurrence of mutations in the laboratory; there are numerous records of the appearance of similarly mutant individuals in nature, but there is little satisfactory evidence that these are the individuals out of which new species are made. Perhaps the best body of proof is that of Crampton's (1909-1928) on the development of geographically isolated races of snails from mutant stocks in various Pacific Islands. In the taxonomic literature there are a few other suggestions of similarly mutant origins of existent species, but such an experienced field worker as Chapman is quoted (H. F. Osborn, 1926) as finding among the birds next to no species which might be interpreted as owing their origin to mutation. Osborn, it may be added, is in accord with this opinion of the minor importance of mutation. Bateson (1922) spoke for many of the geneticists when he expressed the same uncertainty of the application of the laboratory data to species in nature, and recently Anderson (1928) concludes a field and genetic study of two species of iris with the statement that there is little in his evidence to support the mutation theory of the origin of species.
Now, the problem of species obviously goes back to the recognition of the factors which may affect the potentialities of genes. I have nothing to contribute on this aspect of the subject. It is to an increased knowledge of the physicochemical nature of protoplasm and of the gene, and to such experimental work as that of Muller and others on mutations affected by the introduction of measurable amounts of energy, that we must look for the explanation of the first step in the process of evolution. But granting that mutations do on occasion occur, we may present a body of new data to show that these laboratory mutants are precisely the materials which have differentiated many of the species of our genus Cynips.
This evidence becomes available because there are, in the family Cynipidae, more than 70 species of gall makers which have rudimentary or reduced wings strikingly different from the long wings normal among the other seven or eight hundred described species of the family. The differences between these two types of insects are illustrated on several of the plates accompanying this paper. Material on all of these 70 species will be brought together in a later study, but it may be said that all of the data support and extend that part which is here presented on the genus Cynips.
The typical wasp of the family Cynipidae has wings which are somewhat longer than its body. The wings even approach twice the body length (a wing-body ratio approaching 2.00) in certain genera of Cynipidae. The normal wings vary between the different genera and subgenera, but are remarkably constant among the individuals of each taxonomic group. Thus the normal ratio is always about 1.35 in the subgenus Atrusca, 1.50 in the subgenera Besbicus and Cynips, and 1.60 in the subgenus Antron. On the other hand, many Cynipidae have wings less than one-fifth the body length (ratios under 0.20), and there are species in which the wings are completely absent.
If taxonomic classifications may be taken as an expression of an author's conclusions on the evolutionary origins of his group, all previous workers have implied that the short-winged species represent genetic stocks separated from the long-winged species since the day that the first short-winged ancestor came into existence. The genera Acraspis, Philonix, Xanthoteras, Xystoteras, Biorhiza, Parateras, and others have been erected to receive the short-winged gall makers, and the long-winged species have been restricted to genera which contained nothing but long-winged species. It is the familiar story of evolution being conceivable as a function of the remote past, but unacceptable as a matter of moment in the affairs of the present. It is in essence implied that some great cataclysm once upon a time wrought one short-winged cynipid from which all the others have inherited, directly or thru more devious generic paths, all of their unusual characteristics.
There are more than wings to justify these existent classifications. Many of the short-winged species have certain reductions of thoracic characters, enlarged abdomens, often fused abdominal segments, and several other structural peculiarities not recognized among any long-winged species. Thus, the typical Acraspis has a blunt hypopygial spine (figs. 407-429) which is of uniform width for its whole length and terminated by a heavy tuft of hairs, and altho this structure occurs among all the other short-winged species which are Acraspis, this type of spine is not known in any long-winged cynipid. Similarly, the other short-winged genera have been based on groups of characters which would seem to establish their phylogenetic relationships.
Our first doubts of the existent interpretations were aroused when we reached our study of Cynips clavuloides, a common species on the Valley white oak, Quercus lobata, in central California. The typically long-winged agamic female clavuloides (fig. 164) develops in a leaf gall (fig. 142) which more or less resembles a minute Indian club in shape. The form is so distinctive that it naturally brought to mind the only very similar gall known at that time, Weld's species, Xanthoteras teres. Our previous studies (Kinsey 1920-1923) had shown that gall structures are significant measures of an inherited physiologic capacity of these insects. Again, the gall of teres occurs on the leaves of a mountain form of the Oregon white oak (Q. garryana semota) in the southern Sierras adjacent to the Central Valley of California (fig. 28), and all of our experience with the distribution of species had lent support to a corollary of the so-called Jordan's Law, to the effect that the most closely related species occur in adjacent areas. But teres (fig. 162) was a flea-like insect with short wings not a quarter of the length of those of clavuloides, and it had been placed in a genus which contained none but short-winged species. Only the hypopygial spines of the two insects were similar (figs. 188, 190), but we had found that these spines are of great phylogenetic significance; and considering the spines, the galls, and the distribution data, the conclusion seemed inevitable that clavuloides and teres were close relatives.
As we have extended our study to other Cynips, we have repeatedly disclosed similar relationships between many other short-winged and long-winged species, until we are forced to believe that the genus includes 42 subapterous forms which have originated more or less directly from long-winged stocks within the genus. Our bases for the recognition of these affinities are:
1. Close identity of galls, as already explained;
2. Occurrence in adjacent ranges, as follows from Jordan's Law and from our other cynipid data;
3. Close taxonomic affinities between hosts of the insects;
4. Possession of similar hypopygial spines, tarsal claws, and antennal counts, altho we shall show in a later paragraph that dissimilar spines are not evidence of lack of relationships; and
5. Utilization of the bisexual form (where known) as more primitive than the agamic form in these insects (Kinsey 1920:369).
A few typical instances will illustrate our use of these bases. Turn first to the case of the long-winged Cynips acraspiformis, which Weld (1926) recognized as a good species of our present genus, altho he was puzzled to observe that it had a gall (fig. 304) similar to that of a short-winged Acraspis! The map (fig. 59) shows the range of this species. From its distribution and near identity in all structural characters, the closest relative of the long-winged acraspiformis is certainly the long-winged expositor (fig. 340) of eastern New Mexico and. West Texas. The gall of expositor (figs. 301–302) is only slightly different from that of acraspiformis, but the expositor gall is indistinguishable from that of alaria which occurs further to the north. One might collect from West Texas thru New Mexico and into Colorado without realizing he was collecting more than one species, and yet the insect of alaria (fig. 341) is subapterous and was described by Weld (1922) as a good Acraspis. The conclusion seems inevitable. The long-winged acraspiformis and expositor, and the short-winged apache, alaria, calvescens, and villosa are very close relatives, and the short-winged species must have originated directly from long-winged stocks which are still represented in the center of origin of the group (see pp. 66 to 67).
As another instance of the utilization of gall characters, we may cite the connection of the long-winged Cynips nubila with the short-winged pezomachoides. Nubila, occurring in the Southwest, produces a large, wool-coated gall (fig. 299) which is superficially as different from the small, naked, faceted gall of pezomachoides (fig. 312) as one might conceive. However, among the close relatives of nubila is the long-winged acraspiformis which we have just shown is close to the short-winged alaria and villosa. If reference is made to the figures of the details of gall structures of these species, one may find an interesting transition from the galls of the long-winged nubila (fig. 325), expositor (fig. 326), and acraspiformis (fig. 330) to the galls of the short-winged prinoides (fig. 327), erinacei (fig. 328 and 331) and macrescens (fig. 329). The last two of these are naked, faceted galls of the pezomachoides type. Further consideration of the plant tissues which enter into these gall structures (pp. 40 to 43) shows that the same elements are involved in all these galls, and that these elements are so developed nowhere but in the subgenus Acraspis of the genus Cynips. Thus even such superficially diverse galls as those of nubila and pezomachoides evidence the close affinities between short-winged and long-winged species of insects.
In another subgenus, Cynips guadaloupensis, insolens, and patelloides have moderately shortened wings. Weld considered these as species of Acraspis, altho he recognized that the galls are not typical for Acraspis. The three species occur on the canyon white oak (Q. chrysolepis) thruout the mountains of California. In the foothills, on the scrub white oaks (Q. dumosa, Q. durata, etc.) and down in the valleys, on the Valley white oak (Q. lobata) there are numerous long-winged species, echinus (fig. 163), schulthessae, etc., which everyone will accept as typical Cynips of the subgenus Antron. But galls of schulthessae (figs. 151-153) and patelloides (figs. 144145) are remarkably close, with similarities in internal structural details (figs. 192-193) which are infallible indicators of close relationships. The form of internal structure here involved is known nowhere else but in the subgenus Antron. Such gall structures, as well as the adjacent distributions of these species on related oaks, lead us to believe that guadaloupensis and schulthessae are close relatives in spite of their differences in wing characters.
There are a couple of cases of gall identities so thoroly guaranteeing the affinities of dissimilar insects that the systematists have already accepted them. These galls are the large, spherical, thin-shelled oak apples (figs. 262-263) which occur on the leaves of several species of white oaks in the Southwest. There are two stocks, Cynips dugèsi and Cynips bella, with seven described species in the area. The galls of all seven are identical, prolonged studies having failed to show any constant distinctions among them. One may collect galls from the foothills of southern Colorado or the mountains of northern New Mexico and Arizona or from West Texas into southern New Mexico and Arizona and into Central Mexico without finding evidence of more than one species. But upon breeding insects from these galls, each area is found to have distinct species, with fully winged insects in only a few of these areas. Three of the seven insects have wings which are from 15 to 37 per cent shorter than the normal for the subgenus. Weld (1926:18–19) recognized that the species brevipennata with shortened wings “replaced” a fully winged species in the more northern portion of the Southwest, but the contribution which these species offer to the problem of the origin of species needs further emphasis.
The original concept of Acraspis was based on the well-known, short-winged species pezomachoides and erinacei (fig. 3) of the eastern United States. All of the close relatives of pezomachoides are similarly short-winged insects in the agamic generation; but ever since Triggerson's studies (1914) of the life history of Cynips pezomachoides erinacei we should have known what the long-winged ancestors of the group looked like. The experimentally proved bisexual generation of erinacei is an insect with fully developed wings (figs. 1–2), and other characters typical of long-winged Cynips. One can hardly agree with the literature in which these two generations have been placed in different genera. Both of them must represent true Cynips. Here is evidence that long-winged and short-winged species may be nearer than closely related species, for they may be alternate generations of one species.
In several of the cases cited above, we were delayed in our recognition of relationships by the peculiar hypopygial spines which, we have already mentioned, are found nowhere but among short-winged gall wasps. In connection with a generic re-arrangement which we are undertaking for the whole family Cynipidae, we have found that the hypopygial spine is one of the most constant of generic characters among the long-winged insects. This is confirmed by preliminary studies of more than five hundred species of Cynipidae, and the drawings for such long-winged subgenera as Cynips, Besbicus, and Atrusca in the present paper will illustrate our point. For this reason, we were at first reluctant to believe that shortwinged Acraspis or short-winged Philonix, with distinctive spines, were not genera of nearly as ancient standing as previous classifications had indicated. On the other hand, there is one case of an insect, apache, which is obviously a short-winged Acraspis altho it, as far as we can determine from inadequate material, has the same spine (fig. 402) as the long-winged Cynips acraspiformis (fig. 400). Final evidence, however, is to be drawn from such a long-winged, bisexual generation as we have just described for Cynips erinacei where the short-winged agamic form has the peculiar spine (fig. 420) of a short-winged species of Acraspis, and the long-winged bisexual form has the typical spine (fig. 406) of a long-winged species of Cynips. One must conclude that the form of the spine, while thoroly diagnostic among long-winged Cynipidae, is liable to modification among short-winged forms.
In verification of these conclusions, an inverse application of our procedure was called for in the case of Cynips fulvicollis whose small, round, downy galls (fig. 228) are common everywhere in the northeastern sector of the United States. Fulvicollis and its immediate relatives are short-winged insects (fig. 234) which we had concluded were Cynips of the subgenus Philonix, but no long-winged Philonix had been recognized among entomologists. But the bisexual insect, Cynips pallipes, came to our attention as a possible bisexual form of fulvicollis (for reasons given on p. 271), altho pallipes had a peculiar wing-body ratio of 1.17 not then recognized in any subgenus of Cynips, and a hypopygial spine (fig. 251) distinct from that of fulvicollis (fig. 252). There was only one other long-winged Cynips which we had not placed at that time in one of the six subgenera, and that was the agamic C. plumbea of the southwestern United States. Upon reexamination of the data, plumbea proved to have the same wing-body ratio and the same spine as pallipes, while the gall of the agamic plumbea (figs. 225-226) was hardly distinguishable except in color from the gall of the agamic fulvicollis. The peculiar spine of fulvicollis had to be taken as one more case of a modification accompanying wing reduction. Thru galls, alternate generations, wing-body ratios, and interpretations of reduced wings and transferred spines the data became interpretable without contradiction in any part.
For all of the 42 short-winged insects which we are considering as true Cynips there is this same coördination of the evidence. Our conclusions are summarized in the following table, where the style of indentation will indicate the phylogenetic affinities of each form, and the order will show the development from the more primitive to the more specialized members of each group. The number that follows each name shows the average wing-body ratio in that species. It will appear that there are at least 11 stocks of Cynips in which the subapterous condition must have arisen independently, and it is possible that a still larger number of the 42 subapterous species have arisen directly from long-winged ancestors.
Wing-lengths in Agamic Cynips
| 1. Subgenus Cynips | ||
| folii | ||
| folii | 1.50 | |
| bisex. form | 1.50 | |
| flosculi | 1.50 | |
| bisex. form | 1.50 | |
| ilicicola | … | |
| atrifolii | 1.50 | |
| longiventris | ||
| longiventris | 1.50 | |
| bisex. form | 1.50 | |
| forsiusi | 1.50 | |
| divisa | ||
| divisa | 1.50 | |
| bisex. form | 1.50 | |
| atridivisa | 1.50 | |
| agama | 1.50 | |
| disticha | 1.50 | |
| cornifex | 1.50 | |
| 2. Subgenus Antron | ||
| echinus | ||
| douglasii | 1.60 | |
| bisex. form | 1.30 | |
| echinus | 1.60 | |
| bisex. form | 1.30 | |
| vicina | 1.60 | |
| bisex. form | 1.30 | |
| dumosae | 1.60 | |
| mista | 1.60 | |
| schulthessae | 1.60 | |
| bisex. form | 1.30 | |
| guadaloupensis | ||
| guadaloupensis | 0.62 | |
| insolens | 0.80 | |
| patelloides | 0.80 | |
| teres | ||
| clavuloides | 1.60 | |
| hilderbrandae | 0.52 | |
| teres | 0.36 | |
| 3. Subgenus Besbicus | ||
| multipunctata | ||
| multipunctata | 1.50 | |
| indicta | 1.50 | |
| conspicua | 1.50 | |
| heldae | 1.50 | |
| maculosa | ||
| maculosa | 1.50 | |
| tritior | 1.50 | |
| mirabilis | ||
| leachii | 1.50 | |
| mirabilis | 1.50 | |
| 4. Subgenus Philonix | ||
| plumbea | 1.17 | |
| fulvicollis | ||
| rubricosa | 0.38 | |
| vorisi | 0.43 | |
| major | 0.55 | |
| gigas | 0.62 | |
| lanaeglobuli | 0.65 | |
| fulvicollis | 0.55 | |
| bisex. form | 1.17 | |
| canadensis | 0.40 | |
| 5. Subgenus Atrusca | ||
| dugèsi | ||
| simulatrix | 1.35 | |
| dugèsi | 1.15 | |
| brevipennata | 0.85 | |
| pupoides | 0.90 | |
| bella | ||
| bella | 1.35 | |
| congesta | 1.35 | |
| vanescens | 1.35 | |
| cava | 1.15 | |
| centricola | ||
| centricola | 1.35 | |
| clivorum | 1.35 | |
| rubrae | 1.35 | |
| strians | 1.35 | |
| 6. Subgenus Acraspis | ||
| arida | 1.30 | |
| mellea | ||
| rydbergiana | 1.30 | |
| unica | 1.30 | |
| compta | 1.30 | |
| anceps | 1.30 | |
| bifurca | 0.27-0.54 | |
| litigans | 1.30 | |
| concolor | 1.30 | |
| mellea | 0.47 | |
| carolina | 1.30 | |
| crassior | 1.30 | |
| albicolens | 1.30 | |
| conica | 1.30 | |
| nubila | ||
| nubila | 1.30 | |
| russa | 1.30 | |
| incompta | 1.30 | |
| villosa | ||
| acraspiformis | 1.30 | |
| expositor | 1.30 | |
| apache | 0.70 | |
| alaria | 0.32 | |
| calvescens | 0.34 | |
| villosa | 0.30 | |
| consocians | 0.30 | |
| gemmula | ||
| cruenta | 0.27 | |
| fuscata | 0.27 | |
| suspecta | 0.27 | |
| gemmula | 0.27 | |
| bisex. form | 1.30 | |
| pezomachoides | ||
| cincturata | 0.16 | |
| ozark | 0.14 | |
| wheeleri | 0.15 | |
| pezomachoides | 0.23 | |
| derivatus | 0.18 | |
| erinacei | 0.22 | |
| bisex. form | 1.30 | |
| advena | 0.17-0.23 | |
| echinoides | 0.19 | |
| hirta | ||
| undulata | 0.23 | |
| packorum | 0.28 | |
| obtrectans | 0.35 | |
| opima | 0.26 | |
| scelesta | 0.21 | |
| macrescens | 0.21 | |
| hirta | 0.23 |
It seems warranted to conclude that insects differing as thoroly as these long-winged and short-winged Cynips may be among the most closely related species in existence. Or, interpreting this statement, we may believe that diverse species may originate directly—that is, by direct mutation—from each other. It should be apparent to anyone familiar with the laboratory Drosophila melanogaster that these subapterous Cynipidae are quite comparable to the subapterous mutants which geneticists have shown to have arisen by direct mutation in the laboratory from long-winged Drosophila stock (e.g., see the summary publications of Morgan, Bridges, and Sturtevant cited in our bibliography). The explanation that will suffice for Drosophila will probably need no essential modification for the gall wasps, but the Cynipidae may establish the importance of the laboratory mutations as materials from which species populations actually arise.
Among none of the Cynipidae is there evidence that the modified wings have developed by the sort of fluctuating variation and the essentially orthogenetic selection conceived by the Neo-Darwinians. Altho there are many short-winged species which occur directly beside their long-winged relatives, there are no intermediate forms as we might expect from fluctuating variation, with the possible exception of Cynips bifurca which appears to be hybridizing today with the long-winged, parental stock.
There seems no basis for believing the shortened wings or any of the concomitant variations of any adaptive value to any of these insects. The short wings are not confined to warmer or colder climates, and long- and short-winged forms of various species are active at the same season in the same localities. The field data suggest nothing as to the survival value of these outstandingly basic modifications of structure. The evidence is all in favor of believing that direct mutations have occurred as the result of modifications that must ultimately be explained in terms of the physics and the chemistry of genes.
A more detailed genetic interpretation of our material must for the most part be postponed for a later paper; but we may point out that the graded series of individuals, obtained when short-winged species hybridize with long-winged stocks as with Cynips bifurca and several species in other genera on which we shall publish later, indicate that wing characters in these insects may be dependent on multiple factors or perhaps on more than one group of such factors. It also appears possible that other structural peculiarities regularly associated with wing reduction may result from the same mutations of one or two genes in groups of linked genes responsible for wing characters. A single gene mutation in a single generation of insects might then give rise to a very distinct cynipid. Such radically new species are usually placed in distinct genera, and this probably explains why systematists have so often failed to believe that mutation accounts for the origin of species in nature. When the mutations are slight, they pass as products of Darwinian variation. There is, apparently, need of a revision of taxonomic procedure in the light of genetics data.
In conclusion, attention should be drawn to the interesting case of Cynips bifurca, a variable-winged species which we have from only two localities, one in southern Mississippi and one in southern Georgia. Both of these stations, however, are located well within the range of Cynips anceps (fig. 50). The galls of bifurca and anceps are identical (figs. 294–295). The insects have the same hypopygial spine (figs. 389–390), and a peculiar tarsal claw (fig. 350) found nowhere else in the genus except among a few of the close relatives of anceps. The figures of the wings (figs. 357–360) and of the whole insects of bifurca (figs. 338–339) will show that there are two distinct types of wings involved: one which is uniformly reduced, and the other a truncate wing more like the laboratory mutants called “truncate” in Drosophila. The significant thing about the bifurca series is the occurrence of intermediate individuals with wing-body ratios ranging between 0.27 and 0.54, body lengths varying from 2.2 to 3.3 mm., and body color from entirely yellow-rufous in the smaller specimens to darker rufous with some black in the larger specimens. These larger individuals bear a striking resemblance in their wing venation (as far as it is present), body proportions, and much of their color to the still larger insects which are anceps (fig. 337). It is my suggestion that bifurca is the result of recent mutation or mutations which have hybridized with the parental anceps stocks. If the wing characters are actually controlled by multiple factors, we should expect the hybrids to form this sort of graded series. The occurrence of the colonies of bifurca well within the heart of the range of anceps, and the limited extent of these colonies seem indicators of their comparatively recent origins. Bifurca may be an instance of present-day mutation of the sort which, in the past, has given rise to the 42 short-winged species of Cynips.