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Interaction between species in a marine ecosy stem is described by expressions for food consumption and grazing mortality which are consistent with each other and with the Beverlon and Holt model of the population dynamics ofindividual species. A model of primary production is introduced in order to make possible an account of nutricnt circlliation (as examplified by phosphorus) within and nutrient flow through the system. It is demonstrated in an application to North Sea fishencs that recent changes in total yield can be described in some detail under the terms of the model as a function of fishing mortality alone. The composition of the North Sea fauna in the virgin state is discussed and also the conditions under which total yield could be increased above the 1970 level.
Until recently, up to thirteen specics of the scincid genus, Scincus, were recognized, but examination of some 590 individuals frorn a wide range of localities suggests that only three or four are valid. Of these, S. mitranus is confined to eastern and southern Arabia and S. hemprichii probably to southwest Arabia. The remaining forms constitute the S. scincus complex, which may consist in North Africa of two largely allopatric species, S. scincus and S. albifasciatus, although evidence for this is not conclusive. The S. scincus complex is represented in southwest Asia by two forms : S. scincus meccensis in southern Jordan, northwest and west Arabia and S. s. conirostris in southern and eastern Arabia, Iraq and southwest Iran. Scincus appears to have evolved Erom a primitive scincine, very similar to members of the Eumeces schneideri group, especially E. (schneideri) algariensis; it does not seem to be directly related to the sympatric genus Scincopus. Within Scincus, the S. scincus complex is the least specialized component of the genus and both S. rnitranus and S. hemprechii may have been independently derived from it, or from a closely related form. Possibly the whole range of the genus was once occupied by a S. scincus-like species and its distribution was subsequently restricted by the onset of less desertic conditions leaving reduced populations in North Africa, southwest Arabia and southeast Arabia that gave rise to the S. scincus complex, S. hemprichii and S. mitranus respectively. A renewed expansion of arid areas could then have enabled the S. scincus complex to invade southwest Asia. Some of the characters of its most eastern subspecies, S. s. conirostris, may have arisen, or been maintained, by character displacement through contact with S. mitranus.
Sexual reproduction in yeasts has a survival function by providing an alternative to the vegetative processes when conditions are no longer conducive for growth. If both sexes are in the correct physiological state (usually under starvation conditions), then initiation of copulation involves the mutual induction of a sexual response. This response is mediated by diffusible compounds and by physical contact. Initial cell contacts between opposite mating types can be disrupted easily, but stronger intercellular bonds form later that result in the fusion of two cells into one. Union between mates involves mixing of parental gene pools. The new diploid organism or its subsequent offspring might be better equipped to survive in a new environment because they may contain new combinations of parental genes. Hence, sex is more advantageous to the survival of the species than it is to the individual organism. The purpose of this review is to compare the steps in the mating process in three species of yeasts. The various physiological factors, events and regulatory phenomena that are part of the mating process will be described for Hansenuta wingei, Schizosaccharomyces pombe and Saccharomyces cerevisiae in Sections II, III and IV, respectively. Then, in Section V, the similarities and differences among these yeast systems will be discussed. Emphasis in this article will be on recent observations since reviews of earlier work are available for each mating system (for H. wingei: Crandall and Brock, 1968; Crandall and Caulton, 1975; for Schizosacch. pombe: Leupold, 1970; Gutz et at., 1974; for Sacch. cerevisiae: Fowell, 1969a, b; Bilinski et at., 1975; Sena et at., 1975). For a comprehensive review of conjugation in all yeasts, fungi and other micro-organisms, consult Crandall (1977). The three yeasts to be reviewed here are quite diverse in terms of their ecological niches, metabolism, morphology and life cycles. Therefore, for a better understanding of the physiology of sexual reproduction in these organisms, it is necessary first to consider these characteristics. A more detailed description of each yeast is given in Lodder (1970).
Carnian (Upper Triassic) fishes from Polzberg bei Lunz have been known since 1886 but no comprehensive account has been published. Eleven species are described nine of which, Saurichthys calcaratus, Polzbergia brochatus, Peltoplellrus dinlmptus, Habroichthys gregarius, Nannolepis elegans, Phaidrosoma lunzensis, Elpistoichthys pectinatus, E. striolatus and Pholidophoretes salvus are new, and two others, Thoracopterus niederristi Bronn and Gigantopterus telleri Abel, previously little-known. New supraspecific taxa defined are: the order Polzbergiiformes, the family Thoracopteridae and the genera Polzbergia, Nannolepis, Phaidrosoma, Elpistoichthys and Pholidophoretes. Habroichthys. Thoracopterus, Gigantopterus and Nannolepis show an unusual skull-roof pattern and are included in the re-defined order Luganoiiformes. Two new ichthyokentemids considerably extend the known time-range of this family. The genus Pholidophoretes is intermediate between the Archaeomenidae Goodrich 1909, and the Pholidophoridae sensu stricto Nybelin 1966. The Polzberg assemblage was probably mainly marine with a small freshwater contribution; it shows less similarity to the Besano and Raibl assemblages than these do to each other. The Luganoiiformes are probably, but not certainly, monophyletic; relationships within the order are analyzed and a cladogram constructed. The Platysiagiformes, Peltopleuriformes, Luganoiiformes and Cephaloxeniformes could all have been derived from a common ancestor at the Perleidus level and are probably offshoots of the perleidid radiation.