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MITOTIC INSTABILITY IN HAPLOPAPPUS: STRUCTURAL AND GENIC CAUSES

Mitotic instability was found in an intraspecific hybrid of Haplopappus gracilis (Nutt.) Gray and in an interspecific hybrid of H. arenarius Benth. and H. aureus Gray. The latter cross was between distantly related species with difEerent chromosome numbers and amounts of DNA. The intraspecific hybrid exhibited a partly recessive phenotype due to loss of a chromosome segment containing the wild type locus, and the interspecific hybrid showed abnormal developmental patterns for several morphological characters due to chromatin loss. Both hybrids were slower growing, smaller, and generally weaker than parental types. In both examples, this weakness was correlated with chromatin loss due to cleavage by cell wall formation across chromosome arms too long to separate properly at anaphase. This was caused by a very unequal translocation in H. gracilis and to a disparity in genome sizes in the interspecific hybrid. In both examples, the initial chromosome cleavage resulted in a breakage-fusion-bridge cycle that persisted into some BC, progeny of H. gracilis. may lead to a breakage-fusion-bridge (BF]3) cycle (McClintock, 1938, 1939, 1941). Mutations that affiect spindle length or orientation, centromere activation, and differential chromosome replication can lead to single chromosome or whole genome loss. The two examples of mitotic instability found in Haplopappus involve a very unequal translocation in one intraspecific hybrid and genetic control of spindle length in an interspecific hybrid. The two examples are related in that both have one or more chromosomes too long to properly separate at mitotic anaphase. MATERIALS AND METHODS-Intraspecific hybrids of Haplopappusgracilis (Nutt.) Gray (n = 2) and interspecific hybnds of Haplopappus arenarius Benth. (n = 4) x H. aureus Gray (n= 6) were analyzed for mitotic instability after exhibiting certain morphological aberrations in early seedling growth. Seeds of the annual H. gracilis were germinated in tap water and then transferred to waterexpanded Jiffy-7 peat pellets (Jiff9 Products Ltd., Norway) after the seedlings were ca. 10 mm long. Seeds of H. arenarius x H. aureus were surface-sterilized in an aqueous solution of 5% calcium hypochlorite and then rinsed, germinated, or dissected in sterile distilled water. Seedlings or partially developed embryos from mature fruits were then placed on sterile 1% agar made with half-strength Hoagland's solution and grown under lights. Seedlings that reached a length of ca. 10 mm were transferred to water-expanded peat pellets. All seedlings in peat pellets were placed in white MITOTIC instability may be defined as any deviation from a normal quantitative and qualitative division of nuclear chromosomes into daughter nuclei. There are some organisms with genetic systems in which fragments, whole chromosomes, or entire genomes are eliminated during the course of development, and such events are repeated for each new generation (Lewis and John, 1963). Such adjusted systems should not be referred to as instablethey are anomalous. In organisms with a normal mitosis, instability may lead to internal or external deviations that are manifest in a series of abnormal changes. These range from expression of recessive genes in heterozygotes to the complete phenotypic and genotypic expression of only one parent in certain hybrids. The most extreme cases cause lethality. The causes of mitotic instability can be included under two broad categories-structural and genic, but in some examples the two are interrelated. Structural changes that produce chromosome arms so long that they are cut by cell wall formation can lead to mitotic instability (Jackson, 1985). Such a change could be caused by very unequal reciprocal translocations, pericentric inversions, and centric transpositions. Gene-induced structural rearrangements caused by activation oftranspositional elements could produce these kinds of changes plus ring and dicentric chromosomes (Jackson, 1985). All ofthese changes 1 Received for publication 30 October 1984revision accepted 8 April 1985.