THE CONTROL OF SEX IN FUNGI

RAPER, JOHN R. (Harvard University, Cambridge, Mass.) The control of sex in fungi. Amer. Jour. Bot. 47(9): 794-808. 1960.-The control of sexual processes in fungi is imposed at many successive points in the developmental cycle. Two major categories of controlling factors are recognized: (1) a predetermination of future sexual capacity follows from the segregation at meiosis of genetic factors that impose differentiation in respect to sexual sign or incompatibility type; (2) a coordinated progression of interdependent stages, governed by numerous genetic-metabolic factors, leads to plasmogamy, karyogamy, and meiosis. The major features of these 2 types of control--the several patterns of sexuality of the former and sexual hormones and sequential sexual development of the latter-are reviewed, and the possible relationship between the two levels of control is considered in some detail. THE DIVERSITY of sexual processes among the fungi has been, for the better part of a century, one of the more intriguing and more intensively investigated aspects of this group of plants. Hypotheses seeking to explain the control of sexual initiation, progression, and consummation in various forms have been commonplace since the time of Sachs, and during this time the subject has progressed from the generalized concepts of "ferments," "humors," and "potencies" to the demonstrated activities of enzymes, sexual hormones, and genes. Much, however, remains to be done, and it seems worthwhile at this time to summarize past accomplishments and to appraise the field for prospects and promise in the near future. A number of fairly recent reviews on sexuality in the fungi are available (Whitehouse, 1949a, 1951; Raper, 1954, 1959b; Burnett, 1956; Olive, 1958); these and the general literature from which they are drawn reveal sexuality of fungi to be a very complex subject, about which there is no lack of cause for or expression of contrasting interpretative treatments. The control of sex in fungi is achieved in many different ways. These are operative at different successive levels of development and are subject to almost unlimited variation in detail. Two major categories of control may be recognized initially: (1) the basic genetic predesignation of future sexual capacities; and (2) the genetic-metabolic apparatus required for normal development. A third means of control is imposed on the second of these categories by the organism's general physiological status, which, in turn, is subject to profound modification by the environment. The relationships be1 Paper invited by the Editorial Committee. Received for publication February 14, 1960. Cost of excess pagination defrayed in part by author. Previously unreported work on Schizophyllum and Ascobolus contained herein has been supported by grants from the National Science Foundation and by the National Institutes of Health of the U. S. Public Health Service. tween sexuality and nutrition and other environme tal factors that were worked out by Klebs and others about the turn of the century and the more recently demonstrated effectiveness of various specific growth substances (vitamins, purines, amino acids, etc.) apply in a broad sense to the present consideration, but they can be dismissed here on the assumption of a direct correlation between physical health and sexual vigor. An appreciation of the control of sexual processes in fungi, however, can follow only from an understanding of the limits of sexual variability within the group. This variation has 3 major parameters which vary independently to a surprising degree: (1P life cycle; (2) pattern of sexuality; and (3) sexual mechanism. These 3 facets of fungal sexuality have been discussed in an earlier paper (Raper, 1954) and may be briefly summarized as follows. The life cycle defines the order and relative durations of successive nuclear phases. Seven distinct life cycles occur among the fungi: (1) asexual-no change in nuclear phase; (2) haploid; (3) haploid with transient dikaryophase; (4) haplo-dikaryotic; (5) dikaryotic; (6) haplo-diploid; and (7) diploid. Cycles 2-4 all have a single nuclear generation in the diplophase, and cycles 5 and 7 have a single nuclear generation in the haplophase. The pattern of sexuality typically reflects either homothallism, self-fertility of the homokaryotic individual, or heterothallism, obligatory cross-mating between self-sterile, homokaryotic individuals. There are a number of different genetic devices that impose obligatory cross-mating in heterothallic forms either through sexual differentiation or by incompatibility mechanisms. The sexual mechanism describes the mechanical means that account for the juxtaposition of sexually compatible elements and for the eventual fusion of their nuclei. Kniep (1928) recognized 4 basic types of sexual mechanisms: (1) gametic copulation, the fusion of 2 uninucleate, differentiated gametes; (2)