How Ecological and Genetic Factors Interact to Determine When Self-Fertilizing Hermaphrodites of Rivulus marmoratus Change into Functional Secondary Males, With a Reappraisal of the Modes of Intersexuality Among Fishes

Hermaphrodites of Rivulus marmoratus, each in lifelong isolation, were exposed to one or another of the eight combinations of alternative temperatures, salinities, and light intensities, for the first 3-6 months from /4 blastoderm or earlier ("early-rearing period"). For the rest of their lives they were kept in 40% sea water at near 25? C and exposed to natural day lengths except for one artificial short-day season. Their life episodes and the conditions of all the eggs they laid were recorded. Hermaphrodites changed into secondary males in correlation with diminished egg laying and in response to one or another of the four annual, and one artificial short-day seasons, 1961-1965. Onset of this responsivity has a genotype-specific age-dependency. In two genetically unlike, uniparental, homozygous, isogenic groups (clones) of fish, early-reared at high temperature, the fish of one clone changed into males in response to the first two short-day seasons, those of the other in response to the last three, with no overlap. Within the same biotype (clone), this responsivity is advanced calendrically by early-rearing at high temperature; hermaphrodites early-reared at moderate or low temperature very rarely change into males and then only near the ends of lives prolonged by protected conditions. Scheduling of the sex changes was apparently unaffected by salinity, light intensity, or any of three morphoses (nonadaptive modifications), induced by early exposure to dim light and morphosis-specific salinity-temperature combinations. From year to year, the testicular zones of the ovotestes progressively increase in area more than the ovarian zones, expressing visibly the agedependent (time-measuring) factor in sex succession that fixes the onset of of the responsivity to short days. When the ratio of testicular to ovarian tissue becomes large enough, the next short-day season evidently triggers the rapid further proliferation of testicular tissue and the involution of ovarian tissue that ends egg laying, begins secondary-male coloration, and changes self-fertilizing hermaphrodites into functional secondary males by converting ovotestes into secondary testes. Testicular activity, measured by percentages of self-fertilized eggs laid, was much lower throughout the hermaphrodite phase of fish early-reared at low temperature as contrasted with high, but ovarian activity, measured by numbers of eggs laid per unit time, was similar between fish earlyreared at low and at high temperature. Testicular activity rose each year with the increase in day lengths from the annual minimum. Later in the annual daylength cycle, ovarian activity rose in response to the much longer days, and testicular activity was proportionately depressed, i.e. inhibited

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