Contrasting Strategies of Gamogenesis in Northern and Southern Populations of Cladocera

In the northern United States, winter constitutes the chief environmental stress for Cladocera, which anticipate this by entering an intensive and coordinated period of gamogenesis in autumn. Hatching of the resulting resting eggs in spring reestablishes those populations that had died back over winter. At extreme conditions, few or no species remain active over winter, but in progressively milder winter conditions southward, more and more species not only remain active after autumnal gamogenesis but even continue some parthenogenetic reproduction throughout the winter. Toward the south the period of autumnal gamogenesis of these "northern" species becomes gradually less intensive and coordinated and eventually is virtually eliminated. Instead, gamogenesis now tends to occur in spring in Louisiana—Mississippi and Florida, suggesting a shift in the season of major stress for these species from winter to summer. Many of these "northern" species tend to die out in summer or at least become greatly reduced in numbers, possibly because of too high temperatures. Other species, which are mostly very small, tend to take over these water bodies in summer. As they have chiefly sourthern distributions, they are referred to collectively as "southern" species. They tend to become gamogenetic in autumn, and over winter they either disappear completely or else are reduced to very low numbers. Thus, water bodies in florida and the near—coastal region of the Gulf of Mexico tend to be occupied seasonally by two quite different assemblages of Cladocera, a "northern" assemblage in winter, spring, and early summer, which tends to become gamogenetic in spring, and a "southern" assemblage in summer and autumn which tends to become gamogenetic in autumn. Gamogenesis in the south, whether in spring or autumn, is not so well coordinated as autumnal gamogenesis in the north, and with few exceptions it is sporadic and not intensive. The data tends suggest, however, that for natural populations in the south gamogenesis among "northern" species peaks in April to mid—May and among "southern" species in October—November. For the strictly vernal species, gamogenesis tends to occur in March. Still further south in the tropics, gamogenesis occurs very irregularly at low levels of intensity and seemingly without coordination among species and unrelated to any particular season. The resting eggs produced thereby are regarded as insurance against low—probability events that could otherwise eliminate the populations and as a means of continually introducing new genotypes into the population, thereby facilitating short—term adaptation and long—term evolution. Not all the taxa in the southern United States could be given definitive names, as there are many taxonomic uncertainties. Quite a few of the taxa are obviously undescribed, and others are uncertainly equivalent to taxa from elsewhere bearing the same names. There is reason to believe that more north—south species pairs exist than the two already known.