Isozyme Analysis of Plant Mating Systems

Abstract

73 The last decade has seen a marked in,crease in the number of genetical studies of plant mating systems. Earlier detailed work had largely been limited to economic plants in which morphological markers were available. Fryxell (1957) comprehensively reviewed this period and compiled an extensive list of all plant species studied. Each species was classified into one of seven classes according to its predominant mode of reproduction. Classification was based either on genetical evidence (from the segregatillln of morphological marker genes) or on studies of reproductive morphology and development. iTable 3.1 gives the total number of taxa in each class as listed by Fryxell, and shows the diversity of plant mating systems, even at this simpli-fied level. The recent upsurge of research on the genetic analysis of plant mating systems has several bases. One reason is the increased awareness of the diversity of plant mating systems and their central position in determining the population biology of plants. As well, isozyme techniques have provided a wide array of genetic markE;1rs . Allozyme variants have three distinct advan-tages over morphological markers for studies of plant mating systems: (1) allozymes are codominantly expressed; (2) many isozyme loci are highly polymorphic in most populations; and (3) allozymes are unlikely themselves to be su1::Jject to strong selective forces . With the growth of studies on plant mating systems, the diversity of approaches has also increased. The objective of this discussion is to give an overview of the major issues currently meriting attention and provide a guide to the necessary techniques and approaches for pursuing these issues . We will distinguish two broad areas of inquiry, namely that of assessing the mating system of individuals, and that of estimating the mating parameters of populations. The former area has focused on using markers to validate observations from natural history (e.g ., the visita-tion patterns of insect pollinators) and to determine the paternity of individuals, with the aim of detecting variation in male and female fertility among adults. The second major area has deter-mined the effect of mating systems on genotype frequencies in populations following the transi-tion from adults to progeny. Although these areas overlap, they differ from a statistical perspec-tive. In studies at the individual level, greater precision is obtained by increased genetic informa-tion on each individual (i.e. , scoring more loci) even at the expense of sample size. In contrast. more efficient experimental procedures for problems at the populational level usually require the study of a few selected loci on an increased number of individuals (Shaw and Brown, 1982). Table 3.1 Summary of Fryxell's (1957) tabulation of modes of reproduction of seed plants Predominantly unimodal Mixture of modes n 0/0 n 0/0 Outcrossing 832 62 Outcrossing + Autogamous 136 10 Autogamous 226 17 Outcrossing + Apomictic 23 2 Apomictic 125 9 Autogamous + Apomictic 4 0.3 All three modes 2 0.1 n = Number of taxa recorded. 0/0 = Percentage of total determined. 73 The last decade has seen a marked in,crease in the number of genetical studies of plant mating systems. Earlier detailed work had largely been limited to economic plants in which morphological markers were available. Fryxell (1957) comprehensively reviewed this period and compiled an extensive list of all plant species studied. Each species was classified into one of seven classes according to its predominant mode of reproduction. Classification was based either on genetical evidence (from the segregation of morphological marker genes) or on studies of reproductive morphology and development. Table 3.1 gives the total number of taxa in each class as listed by Fryxell, and shows the diversity of plant mating systems, even at this simpli-fied level. The recent upsurge of research on the genetic analysis of plant mating systems has several bases. One reason is the increased awareness of the diversity of plant mating systems and their central position in determining the population biology of plants. As well, isozyme techniques have provided a wide array of genetic markE)rs. Allozyme variants have three distinct advan-tages over morphological markers for studies of plant mating systems: (1) allozymes are codominantlyexpressed; (2) many isozyme loci are highly polymorphic in most populations; and (3) allozymes are unlikely themselves to be subject to strong selective forces. With the growth of studies on plant mating systems, the diversity of approaches has also increased. The objective of this discussion is to give an overview of the major issues currently meriting attention and provide a guide to the necessary techniques and approaches for pursuing these issues. We will distinguish two broad areas of inquiry, namely that of assessing the mating system of individuals, and that of estimating the mating parameters of populations. The former area has focused on using markers to validate observations from natural history (e.g., the visita-tion patterns of insect pollinators) and to determine the paternity of individuals, with the aim of detecting variation in male and female fertility among adults. The second major area has deter-mined the effect of mating systems on genotype frequencies in populations following the transi-tion from adults to progeny. Although these areas overlap, they differ from a statistical perspec-tive. In studies at the individual level, greater precision is obtained by increased genetic informa-tion on each individual (Le., scoring more loci) even at the expense of sample size. In contrast, more efficient experimental procedures for problems at the populational level usually require the study of a few selected loci on an increased number of individuals (Shaw and Brown, 1982). Table 3.1 Summary of Fryxell's (1957) tabulation of modes of reproduction of seed plants