Behaviour of a sex-differential fertility gene in hermaphrodite populations

Abstract

SUMMARYIn genera of flowering plants that contain closely related gynodioecious and near dioecious species, near dioecy has probably evolved from gynodioecy by gradual reduction of seed set on hermaphrodites, caused by the action of several genes for partial female sterility. A literature survey shows that several types of sex-function differential fertility effects are widespread in hermaphrodite plants, so that a model of such variation may be of interest both as a model of normal hermaphrodite populations, and for the above evolutionary process. Such a model is presented where there is a dominant or nondominant gene P for partial female sterility (ovule fertility values w,, w5 < 1), and where partially female-sterile hermaphrodites (" subhermaphrodites ") may be maintained by overdominance for pollen fertility (homozygote pollen fertility value w4 < 1). Subherrnaphrodites have genotypes PP or Pp, and hermaphrodites have pp. Fitness values (numbers of offspring per individual) depend upon genotype frequencies and degree of selfing, so that these values have to be distinguished from the constant fertility or selection values (the w values). Proportions of offspring left through the ovules differ among genotypes, are frequency dependent and differ from those left through the pollen. Outcrossing rates for pollen, t2, are also frequency dependent, differ among genotypes and differ from the ovule outcrossing rate, t5. For t1 t I, t, for hermaphrodites <t5, and for 0< t1 < 1 and 0< w's < 1, this t2 is less than those for subhermaphrodites, which may be > 11. Numerical results are given for equilibrium populations and include some cases of complete female sterility for comparison. These results show that frequencies of subhermaphrodites decrease with increased selfing. The heterozygous subhermaphrodite is the fittest, and the homozygous subhermaphrodite the least fit genotype. The fitness of the heterozygous subhermaphrodite decreases, and that of the hermaphrodite increases, with increased selfing. Except for males, proportions of ovule offspring decrease with increased selfing, and again with this exception and with t5 I, t2 is highest for heterozygous subhermaphrodites, and for this type is > 15, whereas t5 is least for hermaphrodites. The total ovule production of the population increases, and total pollen production decreases, with increased selfing.

Gynodioecy in Plantago lanceolata L. III. Sexual reproduction and the maintenance of male steriles

“…Partial female sterility associated with male sterility has also been found in some cultivated species (Cutter and Bingham, 1977;Tsikov and Nikova, 1980, see also Ross, 1977). However, the decreased seed set appears in most of these cases to be due to a decrease in pollinator activity on male sterile plants (e.g.…”

A Model for the Evolution of Dioecy and Gynodioecy

“…It thus seems most likely that dioecy has usually evolved from gynodioecy. This was originally suggested by Darwin (1877) and has been discussed recently by Carlquist (1966), Ross (1970), Lloyd (1974aLloyd ( , 1974bLloyd ( , 1975, and Arroyo and Raven (1975). This idea is given further support by the fact that the relatives of dioecious species are not infrequently gynodioecious, though gynodioecy is in general a rare condition.…”

“…In a random mating population, androdioecy would seem more likely to evolve than gynodioecy, rather than the other way round, because a decrease in ovule production might more plausibly be thought to give a large increase in pollen output than vice versa. A different explanation for the existence of maleor female-sterility genes segregating in natural populations is heterozygote advantage of these genes (Jain 1961;Ho and Ross 1974;Ross andWeir 1975, 1976). This would be much more effective in random mating than in selfing populations, because there would have to be strong heterozygous advantage in order to maintain such polymorphism in the face of a high rate of selfing (Kimura and Ohta 1971).…”

Sexually antagonistic polymorphism in simultaneous hermaphrodites

“…Several additional models identify general evolutionary criteria for balancing selection in partially self-fertilizing hermaphrodite populations ( e.g. , Gregorius 1982b, 1984; Ross and Gregorius 1983; Ross 1977; Ross 1984; Ziehe 1985; Ross 1985; note, in particular, Gregorius 1982b and Ziehe 1982, who provide general conditions for protected polymorphism in partially selfing hermaphrodites). However, to our knowledge, no model fully explores the conditions maintaining SA polymorphism in a hermaphrodite population, including an explicit analysis of the effects of selfing rates, inbreeding depression, and arbitrary dominance coefficients for each sex function.…”