Mating Types in Euplotes eurystomus
Authors: KATASHIMA RYO 
Published: May 2007

SYNOPSIS. The organism used in the experiments fits Pierson's satisfactory description for Euplotes eurystomus. The mating behavior of 90 stocks of E. eurystomus collected from west Japan was studied. Mating occurs only after a decline in nutritive conditions. When the stocks were mixed together, two at a time, in all possible combinations, mating took place in all the mixtures. Mating occurs in the evening or the morning after mixing. An agglutinative reaction does not precede mating; but before pair formation, characteristic pre‐pairing behavior is observed. Selfing is found in all the stocks, usually after the log‐phase of the growth curve. Culture fluids from various cultures have no detectable capacity to induce selfing, but some of them can suppress selfing. The latter occurs only in specific combinations. Controls mixed with distilled water instead of culture fluid always contain selfers. From the results of such experiments, it is possible to classify the stocks into 9 types, so that stocks of the same type have the same character with respect to suppression of selfing. The culture fluid from type II suppresses selfing only in animals of type I, type IV acts only on type III, VI on V, VIII on VII, and VII on both V and VI. The reciprocal actions also occur, i.e., fluid from I suppresses selfing of type II, etc. The origin of the two mating members of a conjugating pair was traced by making mixtures of normal animals of one type with morphologically abnormal animals of another type. The latter have a distinct swelling at the left margin of the body. Such mixtures (I × II, V × VI and VII × VIII) yield conjugating pairs of normal mated with abnormal animals, i.e. cross‐conjugating pairs, in 86–94% of the animals. Mixtures of type VI with VII yield a smaller percentage (48%) of cross‐conjugating pairs. Mixtures of two cultures belonging to the same type, (II × II, VI × VI and VIII × VIII), in which selfing is not suppressed, yield 42–50% conjugating pairs of normal with normal, (selfing of the normal culture), and 32–45% conjugating pairs of abnormal with abnormal, (selfing in the abnormal culture). Mixtures of two cultures belonging to diverse types in which selfing is not suppressed (I × V, I × VIII, II × III, II × VII, II × IX, III × V, III × VI, III × VIII, IV × VI and VI × IX), yield 36–48% conjugating pairs (selfing) of normal with normal, and 26–50% conjugating pairs (selfing) of abnormal with abnormal. Interbreeding between two types thus normally occurs only when culture fluid of each type inhibits selfing by animals of the other type. These two types, which interbreed freely, may be considered as complementary mating types. The mating types of E. eurystomus thus fall into 5 varieties so that each one (except the fifth) has two complementary mating types. The mating types are numbered I and II in variety 1, III and IV in variety 2, V and VI in variety 3, VII and VIII in variety 4, and only IX in variety 5. Intervarietal mating does not occur in many combinations, but exceptionally it occurs between types V and VII, and between VI and VII. These hybrids become senile and finally die.