Browsing by Author "Hermsen, J.G.Th"

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    Mechanisms and Genetic Implications of 2n-Gamete Formation
    (1984-05) Hermsen, J.G.Th
    Gametes with the unreduced number (= 2h) of chromosomes may originate from different abnormal events during meiosis or arise in apomeiotic somatic ceils of the ovule (apospory). Meiotic nuclear restitution leading to 2n-gamet.es may be associated with fused or parallel second metaphase spindles (First Division Restitution, FDR) or with incomplete second meiotic division (Second Division Restitution, SDR). Its association with failing or reduced homologous pairing leads to 2n-gametes which are genetically equivalent to FDR. The basic differences between FDR and SDR are described and explained. Several mechanisms of 2n-gamete formation occurring in association with failing or reduced homologous pairing, as they occur in megaspore mother cells of apomictic species and in spore mother cells of synaptic mutants, interspecific hybrids, amphimonoploids, and anortho-ploids, are described and discussed. Different mechanisms of 2n-gamete production are compared as to their genetic implications. Data about genetic control of processes leading to 2n-gametes are presented along with a discussion on the effects of environmental factors on occurrence and frequency of 2n-gametes. Index Descriptors: nuclear restitution, 2n-gametes, dyads, apomixis, synaptic mutants, premeiotic doubling, autobivalents, fused spindles, endomitosis, and apospory.
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    Nature, Evolution, and Breeding of Polyploids
    (1984-05) Hermsen, J.G.Th
    Different kinds of polyploids are defined andcharacterized by their genome formulas. It is emphasized that natural polyploids range-in genome composition from true autopolyploids to true allopolyploids. The origin and evolution of polyploids is discussed, and special attention is paid to the role of numerically unreduced or 2n gametes. Methods of inducing autopolyploidy are mentioned along with the requirements that diploid crops should meet in order to obtain useful results from somatic doubling. Methods of resynthesizing existing allopolyploid crops or synthesizing new ones are evaluated and the main requirements for success discussed. It is emphasized that genetically broad initial material is indispensable both for induction of autopolyploids and for (re)synthesis of allopolyploids that are to be useful for breeding. Some characteristic features relevant to the breeding of autopolyploids are described with emphasis on the effects of di-allelism and multi-allelism on inbreeding depression and heterosis. Also some particular characteristics of allopolyploids are treated, and some unconventional breeding approaches are briefly discussed. Index Descriptors: polyploidy, evolution, breeding, somatic doubling, 2n gametes, multi-allelism, heterosis, and inbreeding depression.
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    The Potential of Meiotic Polyploidization in Breeding Allogamous Crops
    (1984-05) Hermsen, J.G.Th
    Meiotic and mitotic polyploidization are compared as to frequency of occurrence in nature and potential for breeding allogamous crops. Meiotic polyploidization via diploid first-division-restitution-gametes is the more powerful approach and can be applied either unilaterally or bilaterally. It is explained how 2n-gametes can be traced by (1) seeking for polyploid plants in diploid populations followed by 4x-2x matings; (2) using colchicine-doubled genotypes in 4x-2x matings; (3) visual discrimination of stained 2n- and n-pollen in diploid plants; (4) establishing the frequency of dyad formation in diploid plants; and (5) large-scale reciprocal crosses between diploids and tetraploids. Basically all mechanisms leading to FDR gametes can be utilized for breeding through sexual polyploidization, because of the largely intact transfer of the parental genotype to the progeny via such gametes. FDR-genotypes mostly produce both 2n- and n-gametes in various proportions. The common occurrence of lethality of triploid zygotes ("triploid block") is explained and its significance discussed. Sexual polyploidization is a desirable breeding technique in those crops which at the polyploid level have a better potential performance than at the diploid level. Examples are given. Such better performance of polyploids is based on the larger potential of non-additive gene effects owing to multi-allelism. The highest degree of multi-allelism is obtained with unrelated highly heterozygous parents, the diploid parents producing FDR gametes. Experimental results are discussed which illustrate the great potential of sexual polyploidization, but at the same time reveal the need of divergent FDR-genotypes with high economic value including resistance to the most important diseases and with a stable mechanism for high-frequency production of 2n (FDR) gametes. An adequate breeding program at the diploid level aimed at breeding such FDR-genotypes is advocated. Index Descriptors: sexual polyploidization, breeding, 2n-gametes, autopolyploids, ornamentals, maize, and combining ability.
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    Some Fundamental Considerations On Interspecific Hybridization
    (1984-05) Hermsen, J.G.Th
    It is shown that the extent to which wild species and primitive forms are utilized in crop breeding depends on the nature of the crop and on the availability and accessibility of the species. Useful genes derived from related species are predominantly those for resistance to diseases and insects, but adaptability, quality traits, and yield also could be improved by including wild species and primitive forms in breeding. Some special applications are the induction of haploids, (re)synthesis of allopolyploids and induction of cytoplasmic male sterility. Gene transfer is usually brought about through normal recombination, but, in cereals, induced recombination and translocation have also been applied. It is emphasized that the normal recombination is most effective. This implies that species that are closely related to crop plants are to be preferred. The breeder should resort to remote species only if they carry unique genes not available in more closely related species. The concept of prebreeding within wild and primitive species before crossing with cultivated forms is explained along with the concept of column breeding for polygenic traits. An integrated breeding procedure based on these concepts is presented. The concepts, though basically correct, need to be critically evaluated as to their practicability in breeding. Pre- and postzygotic barriers to interspecific hybridization are listed along with ways to overcome them. Evolutionary and genetic aspects of interspecific barriers are discussed and data presented on the genetic control of crossability of species. Some experimental data in Solarium and grass species clearly illustrate the nature and extent of various interspecific barriers in remote hybridization. Index Descriptors: interspecific hybridization, crossability, prebreeding, column breeding, interspecific barriers, Solatium, grasses, and introgression.