ABSTRACT
Diplodinium (sub-genus Polyplastron) multivesi-culatum (Dogiel and Fedorowa, 1925) undoubtedly is one of the most interesting representatives of the Infusorian family Ophryoscolecidae (which occurs in the stomachs of ruminants), especially from the systematic and phylogenetic standpoint. The origin of this species shows some peculiar causes which are operative in the development of these Infusorian species, but besides that the present treatise points out some new aspects regarding the development of new species in general.
First of all the systematic position of the species Polyplastron multivesiculatum must be cleared up, because owing to some recent investigations it has become obscure and confused. This species was first discovered and described by Dogiel and Fedorowa in 1925 under the name D. multivesiculatum. In 1927 Dogiel established for this species a new sub-genus Polyplastron. In 1928 Dogiel described from the stomach of Buffelus bubalus L. another species of the same sub-genus, D. (Polyplastron) bubali. In their revision of the genus Diplodinium Schuberg, Kofoid and MacLennan have made Polyplastron an independent genus and have divided it into three species. The first species is the P. multivesiculatum, the second P. fenestratum (Dogiel, 1927) (this form was described by Dogiel only as an aberration of the species P. multivesiculatum), and the third is P. monoscutum (Dogiel’s aberratio confluens). For the species from the stomach of B. bubalus Kofoid and MacLennan have established a new genus with the single species Elytroplastron bubali. In 1933 Becker in a small treatise named the E. hegneri (Becker and Talbott, 1927) (with the following synonymy: D. hegneri Becker and Talbott, 1927, D.-Polyplastron-bubali Dogiel, 1928, D.-Polyplastron-longitergum Hsiung, 1931, and E. bubali Dogiel, 1928; Kofoid and MacLennan, 1932), which already in 1927 he had been the first to describe together with Talbott as a new species D. hegneri, from the stomach of domestic cattle and sheep. But as under this name both authors have described forms having a very heterogeneous structure Kofoid and MacLennan identified some of these forms described by the former authors (under the name D. hegneri) as Ostracodinium obtusum (Dogiel and Eedorowa, 1925). Becker agrees with them, but at the same time points out that some of the forms described by himself under the name D. hegneri can in no case be included in the group 0. obtusum being already identical with the form described by Dogiel in 1928 as the new species D. (Polyplastron) bubali. Kofoid and MacLennan have discovered this form in the stomach ofBosindicus. In view of the law of priority according to the international rules of zoological nomenclature and of what has been said above, the name given by Becker and Talbott must alone be used although the latter’s description wτas less complete than that of Dogiel, and although they have re-examined this form (Becker in 1933: ‘The writer recently re-examined some of the type material of D. hegneri …’) and only subsequently discovered (of course, on the basis of Dogiel’s description) some characteristics formerly unnoticed by them and first described by Dogiel.
It is now most important to know whether Kofoid and MacLennan were right in raising Polyplastron from being a sub-genus of the genus Diplodinium to being an independent genus. They also made independent genera of some other sub-genera, even of the species of the genus Diplodinium. In my revision of the family Ophryoscolecidae (which I am doing now) I shall, of course, touch upon every single case; while in this treatise I shall only put forward such arguments as concern Polyplastron, making them, however, applicable in principle to all other cases. Regarding the seven genera of the family Ophryoscolecidae described up to the present day, Entodinium (Stein, 1858), Diplodinium (Schuberg, 1888), Epidinium (Crawley, 1924), Ophryo-scolex (Stein, 1858), Caloscolex (Dogiel, 1926), Opistho-trichum (Buisson, 1923), and Cunhaia (Hasselmann, 1918), there exist, fortunately, among them such differences that no one can any longer confuse them. Besides, it must be emphasized that all representatives of the genera in question are as a rule so uniform and typical that it is easy to identify them. The chief generic difference is the structure of the ciliary or membranelle zone located in the anterior end of the body near to the mouth. This zone has a structure so uniform and characteristic in all representatives of the same genus that at the first glance through the microscope we may immediately determine the genus. Thus the genus Diplodinium is characterized by two membranelie zones (adoral and dorsal), both of them in the same transverse plane, while between the two zones is found a well-developed processus apicalis, whereby the genus Diplodinium is undoubtedly distinguished from other genera of this family (see Text-fig. 1). The genus Diplodinium was divided by Dogiel into four subgenera (Anoplodinium, Eudiplodinium, 0stra-codinium, and Polyplastron), while Kofoid and MacLennan have raised all these sub-genera into genera proper, adding also some new ones, so that according to them all that formerly was included in the genus Diplodinium is now separated into the following distinct genera: Eodinium, Diplodinium, Eremo plastron, Eudiplodinium, Diploplastron, Metadinium, Polyplastron, Ely-troplastron, Enoploplastron, and Ostracodinium. They made this division by taking the more special characteristics (for example, the skeletal plates) as a criterion for the generic differentiation, which is a great logical error (divisiosit adaequata!). In view of the fact that Ophryoscolecidae have several genera, some with skeletal plates and some without, the skeletal plates cannot properly be taken as a typical criterion for generic division (this would only be possible if the family had only two genera, of which one had skeletal plates and the other not). That this is really so may clearly be seen in the case of Ophryoscolex and Epidinium, because these two genera, so different and so sharply divided, have an identical structure of the skeletal plates. For this reason it is imperative to return to Dogiel’s original division, as Kofoid and MacLennan’s new genera, arrived at by the splitting up of the genus Diplodinium, are nevertheless more closely related to each other than to other genera of the family Ophryoscolecidae. Kofoid and MacLennan’s division has two fundamental errors. The first one is of a theoretical nature. According to their division we make a mistake from the phylogenetic point of view, because all representatives of the phylogenetically uniform genus Diplodinium are being split into independent systematic categories taxonomically equivalent to other genera of the same family which are not so closely related to this group, while on the contrary genera should be equivalent units. The second error is of a practical nature. Related groups are being split, and an entirely unnecessary ballast of new names is created, causing taxonomical and phylogenetical confusion. For this reason it is better to leave all the representatives of this group in one single genus Diplodinium. In so far as there is a necessity for further differentiation, Diplodinium may be divided into subgenera. According to the arguments stated above, and basing ourselves on the facts relative to the systematics of the family Ophryoscolecidae, we are actually forced to make a revision in the sense that Diplodinium sensu latiorebe re-erected as a genus sensu latiore, with sub-genera. One of the subgenera of the genus Diplodinium is Polyplastron, which interests us most here.
After this revision Polyplastron (see Text-figs. 2 and 3) has the following position:
Genus Diplodinium Schuberg 1888.
Sub-genus Polyplastron Dogiel 1927.
Species P. multivesiculatum Dogiel and Fedorowa, 1925.
P. hegneri Becker and Talbott 1927.
The following table shows the hosts of the two species as well as their synonymy.
Before proceeding farther, we must examine why the erection of the species P. hegneri to the new genus Elytroplastron is unjustified and why we are justified, both phylogenetically and taxonomically, in including it in the sub-genus Polyplastron, which after all had already been done by Dogiel. Similarly, we must prove the fallacy of Kofoid and MacLennan’s procedure when making Dogiel’s aberrations of the species P. multivesiculatum, i.e. P. multivesiculatum aberratio confluens and P. multivesiculatum aberratio fenestratum, independent species. Dogiel, to whom we are indebted for a very complete description of P. hegneri, points out that this species in general reminds one of the typical species P. multivesiculatum (Dogiel, 1928:. très semblable au P. multivesiculatum), having, however, distinct features of a separate species (‘… font de P. bubali une bonne espèce’). Kofoid and MacLennan, too, when stating that ‘although the paths of development of the two have been different’, say, however, in regard to P. hegneri and P. multivesiculatum: ‘Polyplastron exhibits approximately the same grade of complexity as Elytroplastron.’ P. hegneri and P. multivesiculatum have the same general shape, the same structure of the ecto- and endoplasm (which is very important!), the same form of the macro-nucleus and micro-nucleus and of the two right skeletal plates; further they have the same shape and position of the rectum and of the anus. The only difference is in the number of accessory contractile vacuoles and in the absence of one of the left skeletal plates. These differences, however, are too small to justify the making of a separate genus. Kofoid and MacLennan themselves, for instance, have left the species Metadinium medium and M. ypsilon in the same genus Metadinium, although the first of these two species shows the two right skeletal plates clearly separated, while in M. ypsilon these two plates are fused (not to mention other notable differences such as the general shape and size of the body, the shape and position of macro-nucleus and micronucleus), and again Kofoid and MacLennan say themselves (1932, p. 60): ‘The skeletal plates are very constant, highly characteristic structures,’ and further (1932, p. 118): ‘In the other genera, the form of the plates is stable within a species and it seems unlikely that Polyplastron alone would be an exception.’ It is evident that, if they had been consistent, they would, in view of the above-mentioned example, also have to divide the genus Metadinium into new and separate genera. If, in view of the great variability of all characteristics including the skeletal plates of the family Ophryoscolecidae, their method were adopted, almost every second species would make a separate genus. Furthermore, the same authors say again (1930, p. 497): ‘The most marked differences are presented by the macro-nucleus, both in position and shape, position of the contractile vacuole, the endoplasmic sac and rectum, and finally the external characters such as shape, size, 1 and the various projections in the form of lobes, spines, and flanges. No one character is sufficient to completely characterize a species and to establish its relationships within the genus. The whole complex of characters must be used.’ Accordingly, both authors contradict themselves not only in the facts but also in their own theses when in one case they put individuals with different skeletal plates (besides other differences) into the same genus (the case of Metadinium), while in another case they split, for the same reasons, two species into two separate genera (the case of Polyplastron and Elytroplastron), which they should never have done, if they had perceived not only the morphological similarity but also the phylogenetic origin of the two species in question (Kofoid and MacLennan, 1932, p. 121: ‘It may be suggested that Elytroplastron and Polyplastron have evolved from a form such as Diplopia s t r o n and in the same direction.’) On the ground of the above-stated arguments it is incorrect to divide these two species into two separate genera, a division into two clearly characterized different species of the same genus Diplodinium, sub-genus Polyplastron, being quite sufficient.
In addition to the typical P. multivesiculatum, Dogiel has described two more aberrations, i.e. confluens and fenestratum, the first one with totally fused right skeletal plates, the second with partly fused skeletal plates. Kofoid and MacLennan have erected these aberrations into separate species: P. monoscutum (instead of using Dogiel’s name confluens, here Kofoid and MacLennan have quite correctly acted in conformity with the Art. 11 of international rules regarding zoological nomenclature, the name confluens having already been occupied) and P. fenestratum. Dogiel has rightly named these forms ‘aberrations ‘, because he seldom found them, in great contrast with the very frequent appearance of the typical species P. multivesiculatum. The aberration confluens Dogiel saw only once! In fact, it is impossible to erect a new species on a single aberrative individual! I had an opportunity myself of examining during a year very rich populations of P. multivesiculatum of Somali sheep, having used every day fresh material from her stomach, and observed them both living and fixed (stained). During several months the P. multivesiculatum, by the number of individuals, was the predominant member of this stomach infusorian fauna. The great number of individuals gave me a wide choice. By careful observation of the forms and the position of the two right skeletal plates (primitiva and carina) in more than 700 individuals belonging to this species, I could see myself that in rare cases the gradual fusion of these two skeletal plates was to be observed. Such fusion shows all possible transitional stages (see Text-fig. 4). In view of the great scarcity of such forms, we cannot speak in this case of any separate form, much less of a separate species; the more so as they differ but slightly from the type, and as there exists a series of gradual transitional stages which show that this form is a simple, individual, aberrative variation. Accordingly, these so-called ‘species’ P. mono.scutum and P. fenestratum must be discarded! Just these aberrant forms reveal to us the direction in the evolution of the family Ophryoscolecidae and its great variability. Dogiel has shown (1925) how this variability is sometimes also conditioned and considerably increased by a special formation of some internal anatomical characters.
Now it is necessary to examine the position of the sub-genus Polyplastron within the genus Diplodinium and to find the line of evolution of the two very significant species P. multivesiculatum and P. hegneri. The complex structure of the skeleton and the vacuolary apparatus of these species must be deduced from one of simpler structure. If all these characters are taken into consideration, there is only one form D. (Eudiplodinium) affine (Dogiel and Fedorowa, 1925) (synon.: D. affine Dogiel and Fedorowa, 1925, after Kofoid and MacLennan, 1932) which is the nearest to Poly-plastron. In this I fully agree with Kofoid and MacLennan, who (as cited above) very correctly noted the relation between these species. From one single, narrow, skeletal plate of the same species of the Diplodinium—sub-genus Eudiplodinium —leads the way to those species of Eudiplodinium which are equipped with two relatively narrow, right-side, skeletal plates, and these are E. affine and E. medium. However, in the case D. (Eudiplodinium) medium tauricum these two skeletal plates that extend from the posterior part fuse into a single one (with transitional stages), leaving anteriorly a greater or smaller opening. We are, therefore, right in assuming that the wide, right-side, skeletal plate of the Diplodinium— sub-genus Ostracodinium (Dogiel, 1927)—has evolved from two narrower ones which had gradually fused into a single one. Dogiel, too, confirms this opinion. In this way E. affine would be a progenitor of Polyplastron and E. medium of the Ostracodinium, as we see in the following scheme:
It follows therefrom that Polyplastron’s position between E. affine and the descendants of E. medium medium, being, however, itself distinguished from the latter as a separate group with orimental development of left-side skeleton preserving, but slightly, their tendency for the fusion of primitiva and carina. Parallel with this growth of the skeletal plate goes the increase in number of contractile vacuoles, as will be shown in the following table:
According to the report of Kofoid and MacLennan and Talbott, P. hegneri was found by them also in domestic cattle; thereby extending our knowledge regarding its occurrence. The only host known before was B. bubalus, according to Dogiel. According to their structure, P. multivesiculatum and P. hegneri belong to the most complex descendants of the family Ophryoscolecidae. They belong to the most evolved forms. From Eudiplodinium, having a constant number of two vacuoles, Polyplastron emancipates itself with a tendency to increase this number. However, it is still a question why this number is increasing. My experimental investigations of a number of vacuoles have led me to realize the fact, which I could also ascertain numerically (for the P. multivesiculatum too), that the total surface and the size of the contractile vacuoles stand in a definite relation to the total surface and the size (Wertheim, 1934) of a certain species of the family Ophryoscolecidae. If we compare the size of the contractile vacuoles of P. multivesiculatum with the size of those of D. (Eudiplodinium) medium, for example, we see how small are those of P. multivesiculatum, and their number ought therefore to be proportionally greater in order to enable them duly to carry on their task. We therefore can establish the rule, that in the family Ophryoscolecidae the number of contractile vacuoles increases only parallel to the increase of that given species, or as well by a simultaneous decrease in the size of some existing vacuoles. This can be clearly observed in the case of the genus Ophryo - scolex as well as with the species O. obtusum (Dogiel and Fedorowa, 1925). While studying the already mentioned abundant populations of P. multivesiculatum, I could ascertain that there were a few atavistic cases in which the forms P. multivesiculatum did not have the usual nine vacuoles but only two dorsal ones; but those were now much larger (in accordance with our above-mentioned rule; see Text-fig. 5.) These vacuoles, owing to their size, were influential in changing the form of the macro-nucleus. This is an analogous case—in a different direction—to that of Dogiel’s (1925). The case of atavistic recurrence in these two vacuoles, typical for Eudiplodinium, is another proof of the fact that Polyplastron was derived from Eudiplodinium. Concerning the skeletal plates, I could ascertain a gradual transition to the Eudiplodinium type with two right-side skeletal plates. Text-fig. 6 shows the orimental development of the left-side skeletal plates as I could ascertain them in the populations of P. multivesiculatum from the Somali sheep mentioned above. Evidently, this enlargement of the skeletal apparatus means intensification of its function. Using chlor-zinc-iodide as a reaction for a stronger brown to black staining of the skeletal plates (reaction on the cellulose) I could observe that with the forms without skeletal plate a collapse would take place at a great concentration of the reagent, while with the forms having right-side skeletal plates it was somewhat slighter, as well as with Ostracodinium, while Epidinium and Ophryoscolex were almost perfectly resistant. This means that the latter are protected by the skeleton, which in this case envelops their body all around. Consequently, regarding the function of these skeletal plates, I can, on the basis of my observations, confirm the following fact: that the skeletal plates have also the function of strengthening the bodies of these forms, not alone of the pharynx. I suppose, owing to a very dense stomach-liquid, the body of the Ophryoscolecidae must be resistant against lateral pressure from all sides. The cuticle, strengthened by a thick layer of ectoplasm (encrusted by silicic acid), has to serve this purpose, but in any case the skeletal plate may intensify this action. It is characteristic that the relative thickness, of course, with regard to the size of the species in question, of the ectoplasm is less as the skeletal apparatus is more developed. To support my assertion I also mention the fact that the skeletal plates usually develop on the right and the left side of these Infusoria (not dorso-ventrally), but as their body is dorso-ventrally flattened they are larger on the right and left sides. Where the surface is larger there is also a greater pressure, so that consequently a greater resistance and hardening of the sides of the body are necessary. According to this, cylindrical forms again (Epidinium, Ophryoscolex) have not a pronounced right-side skeletal apparatus but a cylindrical one, because while in movement their more rounded body is more evenly exposed to pressure of the environmental fluids. The same applies to Caloscolex and 0pisthotrichum. Ostracodinium is in this respect intermediate. If we consider these two representatives of the sub-genus Polyplastron, we shall observe that P. multivesiculatum shows greater complexity than E. affine (compare in Text-fig. 7 the identical parts in the bodies of P. multivesiculatum, P. hegneri, and E. affine). In all probability the form P. hegneri is phylogenetically older. As we can see from our comparative drawings, P. hegneri is closer to E. affine in the position of its dorsal vacuoles, by the form of its macro-nucleus, and by the direction of the right skeletal plates, than is P. multivesiculatum to the E. affine. Thus a cue is given for the tendency of the development of left-side skeletal plates, as shown in the present case of Diplodinium—sub-genus Poly plastron.
Finally the ways in the evolution of the P. multivesiculatum and P. hegneri must be explained, these two species being very interesting by the tendencies in their evolution, and as having a distinctive structure of their skeletons and their contractile vacuoles. These species show how the evolution of Ophryoscolecidae opens up very many possibilities in a most peculiar way. And the two species in question, which in the genus Diplodinium have their own place, have just realized one of those possibilities. They represent a new evolutionary type which in the future can in this direction produce quite independent groups, the more so as I already proved (Wertheim, 1934, Zool. Anzeiger) a ‘caudopetal’ tendency in P. multivesiculatum. Speaking of the factors at work in the evolution of Ophryoscolecidae, it must first of all be pointed out that here the factor of selection in the form of some struggle for life can in no case come into consideration, which is selfevident.
There is only one thing that must be kept in mind: great masses of these forms perish daily with the passing of the food into the omasus and abomasus, where actually their remains may be found. As my former investigations have shown, the species of Infusoria are fairly uniformly distributed in the stomach so that representatives of all species always remain living in a given stomach. Therefore this ‘incidental selection’ would not be decisive. The factor of adaptation to the dense environment is of greater importance, which adaptation in turn favours the development of the skeleton and the caudal projections, and these belong to the most characteristic criteria for the differentiation of the Ophryoscolecidae. Therefore the adaptation here should be regarded as a motive factor in their evolution. Since these Infusoria, however, live in the greatest variety of different environments, i.e. in the greatest number of ruminants’ stomachs, the oecologic factors, too, must be considered. For instance, the same conditions never exist in all those stomachs; the more they differ (on account of different food or some purely physiological causes), the greater is the possibility of differentiation. However, this oecologic factor is a regulative one, not motive, as in all these various stomachs the adaptation as a reaction to all given special circumstances is of primary importance. As far as we know to-day, Ophryoscolecidae are transmitted through contact per os from ruminant to ruminant. This would be in conformity with the already established fact that those groups of ruminants where this contact for any reason was possible in the past but is no longer possible in the present, show a different structure of the species in their infusorial fauna. This is the factor of isolation. It, too, is a regulative factor. The isolation in a narrower sense (in the same locality, mechanical, if on account of difference in size no contact per os takes place; in a small Cameroon goat and a camel, for example, kept in a zoological garden) as well as through geographic isolation may cause a diversity in the several local faunas, simply because infections and reinfections take place through contact per os. Here, in other words, we come to the conclusion that all factors in the evolution are not equivalent at all, that they do not lie in the same dimension, on the same level.
Regarding Polyplastron and its ‘caudopetalitythe structure of the skeleton and of the vacuolary apparatus is a result of the motive factor of adaptation; while the fact that in certain environments some of its varieties are more isolated is a result of oecologic difference or of geographical isolation acting as regulative factors. Thus we can see clearly why P. multivesiculatum was found to be predominant in Europe, the United States of America (primarily), and Northern Persia, while in Eastern Asia, in tropic regions, and in the United States of America 1 P. hegneri is predominant. Besides, P. multivesiculatum is a distinctive inhabitant of the stomach of domestic cattle, sheep, and goats, while P. hegneri is characteristic (besides being present in domestic cattle and sheep) for B. bubalus. Thus the evolution of a certain species depends not only on its particular but also on its other properties, and consequently on the evolution of the species as a whole. And the evolution of the species as a whole is a consequence not of any isolated evolutional factor, but of a number of factors which are interdependent and which act as a unity as cogs in cogwheels. There are great chances of applying successfully to other forms this method of analysing the evolution of species derived from the study of these Infusoria, rightly held by all investigators to be of exceptional interest. For example, Becker observes that they are ‘an assemblage of unicellular organisms unexcelled in complexity and diversity by any other ecologic groups’ (1932, p. 282). Owing to the great importance of these Infusoria it is indispensable to first put in order their systematics as a basis of all further cytological, biological, and physiological investigations.
Summary
In revising the species of the Diplodinium (sub-genus Polyplastron) the author comes to the conclusion that on the basis of the generic criteria in the family Ophryoscolecidae, two distinct genera cannot be erected as was done by Kofoid and MacLennan (the genera Polyplastron and Elytro-plastron). On the contrary it is justifiable to include both of these species in Polyplastron (Dogiel, 1927) as a subgenus of the genus Diplodinium. In conformity with the international rules of zoological nomenclature, the correct names of both species in question are as follows: D. (Polyplastron) multivesiculatum Dogiel and Fedorowa, 1925, D. (Polyplastron) hegneri Becker and Talbott, 1927. Evidence is put forward to show how incorrect is the erection of Dogiel’s aberrations P. multivesiculatum ‘confluens’ (monoscutum) and P. multivesiculatum ‘fenestratum’ into separate species, and how these forms are to be considered only as individual varieties. These views are supported by the author’s own investigations on live and fixed (stained) material.
The author agrees with Kofoid and MacLennan that Po1y-plastron had evolved from forms related to D. (Eudiplodinium) affine, and brings forward arguments resulting from his investigations as to how this relation can be clearly disclosed. He also gives a scheme of the systematic position and phylogenetical relationship of the species in question of Diplodinium—sub-genus Polyplastron.
Conclusions are drawn concerning the relation and kinship of P. multivesiculatum and P. hegneri, as well as the author’s rules in regard to the development of new contractile vacuoles, also the reasons for the development of the skeleton and the position and form of skeletal plates. These skeletal plates serve for a general strengthening of the body besides other functions.
It is argued that adaptation is a motive factor in the evolution (the selection appears as an ‘incidental selection’), while the oecologic factors and the factors of the mechanical and geographical isolation of the ruminant-hosts (in harmony with the per-os manner of infection of the ruminants by these Infusoria) act as regulative factors. These two categories of factors are in diverse dimensions (they are non-equivalent and inadequate). The evolution of characters cannot be considered singly, but only species as units, and this evolution is not a result of isolated factors but of a series of factors permanently acting in reciprocal dependence. The author considers this method of analysis applicable also to other systematic groups.
List of References
The identity of the characteristics enumerated here relating the P. multivesiculatum and P. hegneri having already been mentioned before.
By using chlor-zinc-iodide or a combination iodine-alcohol and con centrated H2SO4 one can get a nice reaction (brown to black) of the skeletal plates.
With a tendency for fusion.
In the United States of America P. hegneri is a secondary immigrant (of Bos taurus).