ALTERNATION OF GENERATIONS IN THE ROTIFER LECANE INERMIS BRYCE

I. LIFE HISTORIES OF THE SEXUAL AND NON-SEXUAL GENERATIONS

¹ From the Zoological Laboratory of the Johns Hopkins University

This paper is the first contribution of a series dealing with the life cycle of the bisexual rotifer Lecane inermis Bryce. The three types of individuals, the mictic females, the amictic females and the males, and the eggs from which they are derived, are described and their life histories are compared statistically. The individuals studied are intrinsically uniform members of the same genetic stock, cultivated under uniform environmental conditions, although they were not all contemporaneous. The comparison is based upon about 100 representatives of each type of individual.

The three types of individuals are characteristically different in length of life. The mean length of life is, for the amictic females, 8.9 ± 0.11 days, for the mictic females, 11.1 ± 0.28 days, for the males, 5.7 ± 0.07 days. This difference is apparent in their life curves. Embryonic mortality is negligible in all three populations. Mortality increases gradually in the male population until the fourth day, then increases suddenly, 83 per cent of the population dying on the fourth to the sixth days. All are dead by the eighth day. All the females survive through the fourth day; thereafter the rate of mortality increases more rapidly in the amictic population. More than half of the mictic females survive the modal life duration, which is the same for both kinds; 12 per cent to 15 per cent survive the longest-lived amictic female and die off very gradually until the twenty-eighth day. The short life duration of the males is undoubtedly correlated with the structural degeneracy, especially of their digestive and excretory systems.

The longer life of the mictic females, as compared with the amictic females, results, in part at least, from the fact that egg-production is for them a less strenuous process. This is evidenced by the following facts. (1) Their male-producing eggs are smaller. (2) The mictic female produces regularly only two-thirds as many eggs as the amictic female. The maximum number is, for the mictic female, 16, exceptionally 17, for the amictic female 24. The mean number of eggs per individual is, for the mictic female 14.2 ± 0.11, for the amictic female, 20.7 ± 0.13. A higher proportion of mictic females produce their maximum number of eggs. The difference between the mode and the maximum is, for the mictics, only one, for the amictics, three. (3) The mean duration of the period of fecundity of the amictic female is 6.4 ± 0.09 days, with a standard deviation of 1.44 ± 0.07 and a coefficient of variability of 22.5 per cent; the mean for the mictic female is 5.1 ± 0.05 days, the standard deviation 0.83 ± 0.04, the coefficient of variability 16.2 ± 0.75 per cent. (4) During this time the mictic female deposits, on the average, one egg every 8.6 hours, the amictic female deposits one egg every 7.5 hours. (5) The amictic females die usually within 24 to 36 hours after deposition of the last egg; 19 per cent of the mictics lived more than six days thereafter. In summary, the mictic female produces in less rapid succession only two-thirds as many eggs as the amictic female, requires for the production of her eggs 1.3 days less than the amictic female and usually lives longer after the cessation of egg-production.

A comparison was made, for the mictic and amictic females, of the degree of correlation between length of life and their fecundity, rate of egg-production and ability to survive the fecund period. Shortlived amictic females are found to have produced fewer eggs than long-lived individuals, in more rapid succession, and to have died within a few hours after the deposition of the last egg, apparently exhausted by the severity of the process of egg-production. The length of life of the mictic female is correlated only in slight degree with the number of eggs produced and the rate at which they are produced.

The mictic female of this species may be fertilized during immaturity or after having produced parthenogenetic eggs. The fecundity of the mictic female is reduced by fertilization, the total number of eggs per individual, in 18 cases studied, not exceeding ten. The maximum number of fertilized eggs produced by a single individual was five. The length of life of the mictic female is not appreciably altered by the production of fertilized eggs.

The differences cited above are evidence of fundamental physiological diversity between mictic and amictic females and males of Lecane inermis.