A molecular cytogenetic analysis of chromosome behavior in Lilium ...

Intergenomic recombination and chromosome translocation in Lilium hybridsIntroductionIntergenomic recombination and chromosome translocation are totally different phenomena,although exchanges of chromosome segments occur in both cases. Whereas recombination isthe result of crossing-over between homo- or homoeologous chromosomes during meiosis (anatural event), chromosome translocations occur due to chromosome aberrations or mutations(Rieger et al. 1976). Among many other differences, recombinations are locus specific events,whereas translocations are random events – implying that any chromosome segment may betransferred to another location in the genome through breakage and reunion. Despite thesedifferences, in recent years the terms recombination and translocation have been used assynonyms in the cytogenetic literature because little is known about the origin of such geneticchanges (Heslop-Harrison 2000; Nicolas et al. 2007; Osborn et al. 2003; Szadkowski et al.2011; Udall et al. 2005). Especially in the case of newly induced polyploids (neo-polyploids)of species such as Brassica napus, intergenomic recombinations have been considered astranslocations that lead to extensive chromosomal structural alterations (Gaeta et al. 2007).Lily (Lilium, 2n=2x=24) species have been used for investigating chromosome structuralalteration by traditional cytogeneticists during the past century. Together with a few otherplant species, especially maize (Zea mays), barley (Hordeum vulgare), wheat (Triticumaestivum) and other crops (Burnham 1962; Lewis and John 1963), chromosomerearrangements such as translocations, inversions, duplications and deletions, have beenextensively investigated. These studies were not only confined to spontaneous events thatoccurred in nature but also included aberrations induced by radiations as well as chemicalagents. In addition to gaining insights into various aspects of chromosome functions andbehaviour, chromosome aberrations were also helpful to establish the relationship betweenchiasma formation and crossing-over (chiasmatype hypothesis). For this purpose, the plantspecies with large chromosomes such as those of lilies are especially helpful because they arefavourable for critical cytological studies. Some of the examples are: inversion heterozygotesin Lilium martagon var. album and L. hansonii in which spontaneous paracentric inversionswere used to test chiasmatype hypothesis (Richardson 1936); x-ray induced terminal deletionand paracentric inversion in L. formosanum were used to establish the relationship betweenchiasma and crossing-over (Brown and Zohary 1955); using a reciprocal translocation in L.maximowiczii, chiasmatype hypothesis was confirmed (Noda 1960); by analysing a pair ofheteromorphic chromosomes resulting from reciprocal translocations in Disporum sessile,chiasmatype hypothesis was also confirmed (Kayano 1960). A favourable feature ofreciprocal translocations in the case of L. maximowiczii and Disporum sessile was thepossibility to quantify the frequencies of chiasmata in the interstitial segments (chromosomesegments that lie between the centromere and the translocated segment) by estimatingequational and reductional separation at anaphase I stages (see later). In all the above casesanalyses of meiotic stages in pollen mother cells have been successfully used.33