Significance of Sexuality

Emile François MAUPAS. Théorie de la sexu-alité des infusoires ciliés, in Comptes Rendus de l’Académie des Sciences, vol. 105, No. 7, p. 365, Paris, 1887; tr. by T. S. Hall for this volume.

In my previous communication I tried to give a complete formula for the morphological phenomena accompanying the conjugatory coupling of the Cili-ates. I review them here as briefly as possible.

The micronucleus constitutes an hermaphroditic sexual mechanism. It is the only organ whose activity plays a role essential to conjugation. First, it passes through a growth phase, A, followed by two phases of division, B and C, culminating in the elimination of excess bodies. These phases, B and C, correspond, then, to the two divisions of the germinal vesicle which effect the elimination of polar globules in metazoa. Phase D, which follows immediately, is another division phase producing the differentiation of a male pronu-cleus and a female pronucleus. During phase E, the conjugants carry out a reciprocal exchange of their male pronuclei which unite and fuse with the female pronucleus of their new host, thus constituting a new nucleus of mixed origin. Here the essential part of fertilization ends. The two phases of division which follow have as their end the reestablishment of nuclear dualism peculiar to the Ciliates. Finally, during the last phase, H, the reconstitution phase, the exconjugants reassume their normal structure and organization, and then undergo their first fission. The old nucleus has been deorganized and eliminated by resorption.

These definite facts having been ascertained, what, now, is their physiological meaning? Since the beautiful work of Engelman and of Bütschli, we know that they are not followed by any production of new individuals distinct from the exconjugants. Hence, authors who have spoken and still speak, of this as sexual reproduction make an obvious mistake.

The observations according to which a supposed increase in fission occurred after conjugation seem to me to prove nothing. I have isolated, immediately after conjugation, individuals of several species. During successive generations of descendants, they underwent fission without showing the least acceleration.

One could even support the idea that, far from contributing to the multiplication of Ciliates, conjugation is one of the most active causes of their destruction. During conjugation and particularly during the long period of inactivity characteristic of the reconstitution phase, they are much more exposed to risks and dangers of the struggle for existence. Further, while not conjugating, they would have gone on dividing: e.g., an Onychodromus grandis would have produced from 40,000 to 50,000 descendants within the time elapsing during a simple conjugation ending with simple division into two. One thus would not speak of conjugation as necessary and inevitable; from all my experiments it comes out, quite the other way, that the Ciliates, at periods of sexual maturity, couple only when stimulated by special conditions which it would require too long to describe here.

But, whether or not the conjugation is a cause of the destruction of individuals, it is at any rate an indispensable factor for the conservation of the species; and this is, I believe, its sole use. This conclusion comes out of the following experiments.

On November 1, 1885, I isolated a Stylonichia pustulata and placed it in the usual culture. I observed and recorded the uninterrupted generations of its descendants until the end of March 1886, at which time this culture was extinguished by exhaustion of the strain, the individuals having lost the faculty of feeding and reproducing. The number of asexual generations during the total existence of the culture was 215. Individuals which I withdrew from it and allowed to intermingle with descendants from a progenitor of foreign origin supplied me with numerous instances of conjugation.

March 1, 1886, I isolated an exconjugant provided by one of the intermin-glings just mentioned. Its culture, followed through and watched like the preceding one, lasted till July 10, the period when it was likewise extinguished by exhaustion of the strain, after an uninterrupted series of 315 fissions. During the entire period, I effected numerous interminglings with foreign individuals. From these interminglings, I obtained numerous couplings beginning with the 130th generation. The couplings were successful and the exconjugants which emerged were normally reorganized. On the other hand, individuals nearly related and not intermingled, which had lived together without conjugating up to the 180th generation from then on conjugated frequently. But all these latter conjugations were unsuccessful, the exconju-gants dying off slowly without regaining their normal organization.

I likewise followed through to the period when they became exhausted, similar cultures of Onychodromus grandis, two of Stylonichia mytilus, one of

Leucophrys patula and one of an Oxytrich of unknown species. Extinction occurred around the 330th generation with Onychodromus, around the 320th with Stylonichia, around the 330th with Oxytrich, and around 660th with the Leucophrys. In the nonintermingled preparations from these long cultures no coupling occurred; while in preparations involving withdrawal and admixture with foreign strains, I obtained many, in the case of the Onychodro-mus and the Leucophrys. For a reason which eludes me Stylonichia mytilus absolutely refused to conjugate. I had no foreign Oxytrichs to use in effecting admixtures.

The obvious conclusion from these long and tiring experiments is that the life of a strain is with Ciliates arranged in evolutive cycles each having as its point of departure an individual regenerated and rejuvenated by a sexual coupling. This result takes us back to an interpretation of conjugation such as has already been given by Bütschli. Sexual fecundation, which previously we visualized as so indissolubly linked with reproduction, has with the Ciliates remained quite distinct and independent. Reproduction here is always aga-mous, while sexual fertilization causes a simple rejuvenation, a reorganization of the individual conjugants. The reorganization manifests itself especially and probably uniquely in the nuclear mechanism. The latter, when the series of agamous generations is unduly prolonged, undergoes a degeneration and disorganization which I shall describe elsewhere. If conjugation does not intervene in time to check the destructive effect of this degenerescence, death inevitably ensues.

This is the true natural death from old age declared by certain authors not to exist among Protozoa to which they attribute a presumed immortality imposed upon eternal youth.

^* For biography see Bulletin de la Société d’Histoire Naturelle de l’Afrique du Nord, vol. 7, 1915.