Die Geschlechtsbestimmung Des Flohkrebses Gammarus Duebeni Lillj. (Amphipoda) Ist Temperaturabhängig - Eine Entgegnung

This paper discusses the effects of physical factors, especially temperature, on sex ratio and sex determination in some euryplastic amphipods and isopods. In particular, it subjects to critical analysis a recently published article by Traut (1960) which purports to prove that temperature does not affect sex ratio and sex determination in the amphipod Gammarus duebeni Lillj. Traut comes to the conclusion that "a significant effect of temperature can be excluded with certainty" (: 454; the present author's translation), and that sex determination is genotypical and practically independent of the environment (polyfactorial sex determination). This conclusion is not in accord with my earlier results (Kinne, 1952a, b, 1953a), ecological considerations (: present paper) - or Traut's own data (Table II). The present paper presents (1) a brief review of the techniques and conditions employed in my experiments, and of my results; (2) proof that Traut used a different technique and exposed his animals to different conditions - circumstances which a priori had to lead to different results; (3) proof that Traut misinterprets his data, which, in general, confirm that the temperature affects sex determination; (4) a discussion of the ecological significance of an environment-dependent sex determination versus a genetically fixed "monogeny". The sex ratio and sex determination of G. duebeni is controlled to a large degree by temperature, to a much lesser degree by salinity (Kinne, 1952a, b, 1953c), and possibly also by other environmental factors. Females kept at a constant salinity of 10 ± o.4‰ and under normal annual temperature fluctuations (minimum 1-2° C, in January; maximum 23° C, in July, August) produced males if the temperature during the "sensitive phase" (a certain number of days previous to ovi-position ; Kinne, 1953c : 266) happened to be below 5° C, they produced females if this temperature happened to be above 6° C, and mixed broods if it happened to be between 5° and 6° C. The "critical temperature range" (5-6° C) may change with adaptation to different environmental conditions (Kinne, 1952b, 1953c); it is, for example, higher and wider in warm adapted (19-20° C) females, and also in females adapted to higher (30‰) salinities. Examples are presented to elucidate the importance for survival and ecological success of a plastic, environment-dependent sex determination in G. duebeni. Traut exposed his G. duebeni to only two different temperature conditions: a constant temperature of 4° C and "room temperature" (he gives no information on salinity, light, food, etc.). Under these conditions he finds that certain females ("arrhenogen") produce only males, others ("thelygen") only females. This "monogeny" is, according to Traut, genetically fixed. The difficulties of explaining monogeny on a purely genetical basis (as attempted in the genetical-polyfactorial theory of sex determination: de Lattin, 1949, 1950, 1951, 1952; Seitz, 1953, 1954; Anders, 1953, 1957; Traut, 1960), the various exceptions observed (e.g. "thelygen" females suddenly producing males, etc.), and the criticisms of the assumed polyfactorial sex determination theory (Hartmann, 1956, and others) have led the present author to suggest a different interpretation of the phenomena observed by de Lattin and his pupils. It is suggested that it is not monogeny itself but rather the range of the critical temperature or, more generally, the physiological reaction norm ("Umwelteinstellung") of individuals that is genetically fixed. Arrhenogen and thelygen females would then represent individuals with different, genetically fixed ranges of critical temperature. Genetically fixed differences of this sort are found in various species (temperature races, climatic races; see e.g. Precht et al., 1955 : 170-176), and indeed are sometimes deliberately enhanced by selective breeding, e.g. in food plants and domestic animals. On the basis of the pertinent experimental results and the assumed genetic differences in the "Umwelteinstellung" of individuals, the following would seem to be the most important factors controlling a plastic environment-dependent sex ratio (for simplicity, reference is made to temperature only) : (1) Temperature-dependent sex determination with the critical temperature range capable of "physiological adaptation" (in the sense of Prosser, 1958) (2) Temperature-dependence and adaptability of secondary processes: timing of reproductive and resting periods; molting frequency; ovi-position frequency; growth rate; egg production; incubation time; duration of precopula (Kinne, 1952b : 130) (3) Selective mortality (4) Genetically fixed differences in the physiological reaction norm ("Temperatureinstellung") of individuals (see e.g. Precht et al., 1955 : 171); mutations; selection of genetically suitable material.