Haploidy from Hordeum interspecific crosses

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Abstract
Interspecific crosses of Hordeum parodii (2n = 42) with H. bulbosum (2n = 14 or 28) and H. vulgare (2n = 14;, and of H. proaerum (2n = 42) with H. bulboswn, H. vulgare and H. parodii were made. Crosses between parodii and diploid bulbosum resulted in haploids (2n = 21) of parodii, whilst the crosses of parodii by tetraploid bulbosum or diploid vulgare gave hybrid progeny. The procerum by diploid bulbosum cross invariably produced haploids (2n = 21) of procerum, whereas procerum by tetraploid bulbosum or diploid vulgare crosses resulted in both hybrids and haploids of procerum. The cross between procerum and parodii gave hybrid progeny which did not reach maturity. Cytological observations on two-week-old embryos obtained from reciprocal crosses revealed chromosome variability (not less than 21 in any cell) in haploid producing crosses. This shows that chromosome elimination leads to haploid formation irrespective of which species was used as female parent. The results indicate that the ratio of the parental genomes in the zygote determines whether predominantly haploids or hybrids will be produced in any cross combination. Furthermore, procerum appears to be not only more efficient in eliminating bulbosum chromosomes in comparison with parodii, but also capable of eliminating vulgare chromosomes. The possibility of ‘stability factors’ in overcoming chromosome elimination, a hierarchy of chromosome elimination and the general existence of genome balance for chromosome stability in interspecific crosses, are discussed.