II. 4 Genetic structure of Hordeum spontaneum in Israel.
A.H.D. Brown, CSIRO, Division of Plant Industry, Canberra, Australia: E. Nevo, Institute of Evolution, Haifa; and D. Zohary, Laboratory of Genetics, Hebrew University, Jerusalem, Israel.
Genetic variation stored in the wild progenitors of cultivated crops represents a major resource for future breeding. Yet the amount, and spatial patterns of genetic polymorphisms in populations of the wild relatives are still largely unknown; especially that portion of variation which is implicated in adaptations to various climates, soils, disease resistance etc. Recently the technique of gel electrophoresis has permitted the screening of one mode of genetic variation in plant populations. Using this technique, we aimed to measure the allozyme variation in populations of Hordeum spontaneum, the wild progenitor of barley, and test its biological significance.
Loci and genetic variants encountered in Israel
Electrophoretic procedures were developed to assay 28 genetic loci on fresh seedling leaf and flooded roots. The loci scored in leaves were Acid phosphatase (Acph-1, 3), Aldolase (Ald), Catalase (Cat), Esterase (Est B,C,D), Aspartate aminotransferase (Got-1,2), Malate dehydrogenase (Mdh -1,-2), Diaphorase (Nadhd-1,-2), Dipeptidase (Pept -1,-2), Phosphoenolpyruvate carboxylase (Pepc), Glucosephosphate isomerase (Pgi), Phosphogluconate dehydrogenase (6 Pdg-1,-2), Tetrazolium oxidase (To-1,-2), and General protein (Gp). The roots were scored for Acph-2, Alcohol dehydrogenase (Adh-1,-2), Est A, Glutamate dehydrogenase (Gdh) and Phosphoglucomutase (Pgm). A total of 77 variant alleles were found at these loci, which individually ranged from invariant (Ald, Pepc, and To-1) to highly polyallelic (Est B with 15 alleles; Acph-2, 9; Est C, 7). Segregation observed in progeny from naturally occurring heterozygotes was according to Mendelian expectations with the possible exception of Gp.
Many of the variants were common so that the species is highly polymorphic. Thus, the probability that two plants chosen randomly from a bulk of these Israel populations would carry different alleles at a locus was 19% on average. About half the variation was "within populations", because the average probability of allelic difference if the two plants came from the same population (the diversity index of Nei, 1973) was 9.8%. The remainder of the variation arose from differentiation between populations, in the kinds of alleles they contained, or in their frequencies of occurrence.
Levels of genetic diversity in natural populations in Israel
The samples were collected from 28 populations representative of the remarkably wide environmental range of the species in Israel (Nevo et al, 1978). The locations ranged in altitude from -200 to + 1,400 metres, and in mean annual rainfall from 100 mm (scattered, favoured sites within steppic areas), to 1,000 mm (the wettest areas in Israel). There was a wide range in the level of allozyme diversity from one population to another by various criteria. The mean number of alleles per locus varied from 1.0 to 1.79 (average 1.48), the probability of polymorphism at a locus varied from O to 0.43 (average 0.30), and the diversity index varied from 0 to 17.6 (average 9.8%). In a comparable sample from the 17th generation of barley composite cross XXI, which is regarded by breeders as highly variable, these figures were 1.43, 0.25 and 6.7% respectively. Thus the average population of H. spontaneum in Israel is about 50% more variable than this late generation of the composite. The observed proportion of heterozygous loci was 0.3% in the wild populations and 0.4% in the composite. Both are very low due to predominant self fertilisation.
Outcrossing rates and the evolutionary status of H. spontaneum
Quantitative estimates of the mating system parameters are crucial to an understanding of the evolution of plant populations. Based upon flavonoid patterns, but without data on outcrossing, it has been suggested that natural populations of wild barley in Israel have been heavily introgressed by cultivated barley grown nearby in recent times, and that "true" H. spontaneum probably does not exist (Frost and Holm, 1975). If this were correct, then the genetic resources of wild barley would, in practice be inferior to those of cultivated strains, because they would offer little that was not already available in better, more domesticated genetic backgrounds.
The studies of protein polymorphism revealed remarkably high levels of genetic variation within and between populations of wild barley. If recent introgression were to be the major source of such diversity, evidence of a very open and variable mating system would be expected. Our estimates of the rate of outcrossing (as opposed to self fertilisation), however, indicated the contrary (Brown et al, 1978). The overall average was 1.6% outcrossing. There was some variation in this level in that the average estimate from populations in the xeric region (0.4%) was four times lower than that the mesic regions (2.1%). We concluded that such rates could not account for the current level and range of genetic diversity in populations of wild barley. It follows that these populations offer considerable and probably novel genetic resources to the breeder, despite their genetic background being less domesticated than cultivated material.
Association of alleles at Esterase loci
The aim of genetic conservation is to collect and preserve representative samples of genetic variation from the primitive "land races" and the related wild species of the plants by which we live. These activities are open ended, costly, difficult to organise and in some cases, already impossible because the genetic resources have been lost. Therefore it has been argued (Brock, 1971) that breeders might obtain the genetic variation they need at will by mutagenesis, and that less emphasis should be placed on conservation. Such a counter proposal is defensible in the case of single loci. However it is extremely unlikely that populations treated with artificial mutagens could replace natural gene pools as a source of coadapted gene complexes. Such complexes can be defined as genes held together in adaptive sets by linkage, by structural or numerical chromosomal hybridity, by the breeding system and/or by selection. Just how predominant are co-adapted complexes as a feature of population genetic structure is a key question in current population biology.
The esterase loci in cultivated barley have been intensively studied by Allard and colleagues (Allard et al, 1971). There is considerable evidence to indicate that the three tightly linked loci behave as a complex. Only two specific, complementary arrays of alleles at these loci are favoured in composite crosses (Clegg et al, 1972).
These loci are polymorphic in nearly all Israel populations of wild barley studied so far. Furthermore, the populations exhibit only a restricted number of complementary arrays of alleles. Thus there are highly significant associations of alleles at these loci, that is, there is marked linkage disequilibrium (Brown et al, 1977). Presumably, these associations are built up and maintained in part by natural selection. Migration, drift and the relative isolation of lineages (due to self fertilisation) would also play an episodic role. The occurrence of complexes of alleles in natural populations thus strengthens the argument that we should collect and conserve the genetic resources of crop plants.
References:
Allard, R. W., Kahler, A. L. & Weir. B.S. 1971. Isozyme polymorphisms in barley populations. Barley Genetics 2:1-13.
Brock, R.D. 1971. The role of induced mutations in plant improvement. Radiation Botany 1:181-196.
Brown, A.H.D., Nevo, E. & Zohary, D. 1977. Association of alleles at esterase loci in wild barley. Hordeum spontaneum. Nature 268:430-431.
Brown, A.H.D., Zohary, D. & Nevo, E. 1978. Outcrossing rates and heterozygosity in natural populations of Hordeum spontaneum Koch in Israel. Heredity (submitted).
Clegg, M.T., Allard, R.W. & Kahler, A.L. 1972. Is the gene the unit of selection? Evidence from two experimental plant populations. Proc. Natl. Acad. Sci. U.S.A. 69:2474-2478.
Frost, S. & Holm, G. 1975. Variation of flavonoid patterns in Hordeum spontaneum and H. agriocrithon. Hereditas 80:167-172.
Nei, M. 1973. Analysis of gene diversity in subdivided populations. Proc. Nat. Acad. Sci. U.S.A. 70:3321-3323.
Nevo, E., Brown, A.H.D., & Zohary, D. 1978. Genetic variation in the wild progenitor of barley. Nature (submitted).