GrainGenes

II.2. Evidence of cytoplasmic evolution from matings between barley and Elymus.

H. Ahokas. Department of Genetics, University of Helsinki, P. Rautatiekatu 13, SF-00100 Helsinki 10, Finland.

Intergeneric hybrids of Hordeum vulgare s.l. and Elymus arenarius were obtained by cultivating embryos in vitro on a nutrient medium (Table 1). When E. arenarius is used as the pollen parent, one frequently succeeds in getting hybrid embryos. The reciprocal cross, though tried approximately to the same extent, resulted only in two well-formed embryos that were able to develop further. Both of them gave rise to a small, apparently hybrid seedling, but lost its barley chromosomes through elimination during a lag in the growth. Subsequently, monoploids of E. arenarius with 28 chromosomes arose. One of them is about three years old, and the only spike of this plant was sterile. The growth of the other older one has been described previously (Ahokas 1970).

Table 1. Number of hybrids and maternal monoploids from matings between barley and E. arenarius.

Since then, in the summer of 1971, the latter monoploid E. arenarius set about 25 ears. A segment of the tuft, ca. 30 cm in diameter, bore three almost totally male-fertile and highly female-fertile ears. All the parts of the fertile shoots were of the same size as those of the sterile ones, e.g., the flowers and kernels (Figure 1). The fertile ears were selfed, and in addition, the pollen was used in crossing. The selfed kernels gave rise to seedlings with 56 chromosomes, which is the number of the euploid E. arenarius. The mode of the reversion has not been studied yet. It can be caused (1) by a somatic reversion to the 56 chromosome level, or (2) a mutant consistently giving a high frequency of unreduced gametes (dyads). Both are expected to give a homozygous offspring.

Figure 1. Kernels of E. arenarius. - A. Empty hulls from a sterile spike of the monoploid plant. - B. Well-formed kernels from a fertile spike of the monoploid plant. - C. Kernels of a normal plant. - All are natural sized.

During the in vitro culture, the major part of the barley (female) x E. arenarius hybrids turned inviable rosettes (Ahokas 1970). They had the waxiness and stiff stature of Elymus. Therefore, they might have been monoploids of E. arenarius in the barley cytoplasm. The true, viable hybrids have remained vegetative, setting no ears after wintering or cold or gibberellin treatments. Most of the hybrids usually perish in our winter conditions.

The distribution of hybrids and monoploids from the reciprocal crosses (Table 1) can be expected to be at P = .022, according to the Fisher exact probability test. The 14 hybrids involve seven which have the fertile revertant of the monoploid E. arenarius as the pollen parent (Table 2). Thus, the genome of the monoploid successfully hybridizes in the barley cytoplasm.

Table 2. Hybrids (= H) and maternal monoploids (= M) from the different crosses.

The results imply that the barley and E. arenarius chromosomes are incompatible in forming a hybrid in the cytoplasm of E. arenarius, but they, in many combinations of the genotypes of these species, choose each other in the cytoplasm of barley, though the hybrid cell contains 35 chromosomes, which is 2.5 times that normally present in the barley cell. The unilateral cytoplasmic incompatibility can be considered to be evidence of the cytoplasmic evolution, as Hordeum and Elymus probably had a common prototype in the past.

Reference:

Ahokas, H. 1970. Ann. Bot. Fenn. 7:182-192.

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