Sex and clonal longevity

The basic question of whether or not conjugation can extend the finite lifespan of a vegetative clone was settled by Calkins in 1919. Starting with a single individual of Uroleptus mobilis, he obtained exconjugants from crosses within the original parental line (clonal self-fertilization) or within or between the derived lines. These exconjugants were therefore rather highly inbred. Each line when propagated without conjugation had a finite lifespan of some 200–300 days, with a steep decline in fission rate during the latter part of this period (see Table 2 and Figure 5). The exconjugant lines, however, usually outlived their parent line, making it possible to maintain the organism in culture (Figure 15). Conjugation, therefore, restored a youthful fission rate and was essential for the continued existence of the stock. Figure 15 is one of the more important experimental results in biology, since it leaves no room for doubt that sex does indeed possess the rejuvenatory property so long claimed for it.

This result was confirmed by Woodruff and Spencer (1924), using Spathidium spathula, again a species which almost always shows a decline in fission rate culminating in extinction when cultured asexually (Table 2 and Figure 5). Like Calkins, they began with a single individual from which all their lines were ultimately derived. Their data are not quite straightforward to analyse, since many lines were discontinued before they became extinct; using only those lines which died out during the period of observation will therefore underestimate the true longevity of the derived lines, while using only those which were discarded before extinction will yield an overestimate.