Comparative mapping of loci controlling winter survival and related traits in oilseed Brassica rapa and B. napus C. Kole 1,2 , C.E. Thormann 1 , B.H. Karlsson 3 , J.P. Palta 3 , P. Gaffney 4 , B. Yandell 3,4 and T.C. Osborn 1, * 1 Department of Agronomy, University of Wisconsin, 1575 Linden Drive, Madison, WI 53706, USA; 2 Current address: Division of Genetics and Plant Breeding, Allahabad Agricultural Institute, Allahabad, 211007, India; 3 Department of Horticulture, University of Wisconsin, 1575 Linden Drive, Madison, WI 53706, USA; 4 Department of Statistics, University of Wisconsin, 1210 W. Dayton St., Madison, WI 53706, USA; *Author for correspondence (e-mail: tcosborn@facstaff.wisc.edu; fax: 1608 262 5217) Received 26 September 2001; accepted in revised form 6 February 2002 Key words: Brassica napus, Brassica rapa, Flowering time, Freezing tolerance, Genome homology, Quantitative trait loci, Winter survival Abstract Winter survival is an important characteristic of oilseed Brassica that is seeded in the fall in northern climates, and it may be affected by genetic variation for other cold-regulated traits, such as freezing tolerance and vernal- ization responsive flowering time. We analyzed immortalized populations of oilseed Brassica rapa (recombinant inbred lines) and B. napus (double haploid lines) derived from crosses of annual and biennial types in order to compare the map positions and effects of quantitative trait loci controlling winter survival, nonacclimated and acclimated freezing tolerances, and flowering time. The B. napus population was evaluated in multiple winters, and six of the 16 total significant QTL for winter survival were detected in more than one winter. Correspon- dence in the map positions of QTL controlling different traits within species provided evidence that some alleles causing greater acclimated freezing tolerance and later flowering time also contributed to increased winter sur- vival. Correspondence in the map positions of QTL between species provided evidence for allelic variation at homologous loci in B. rapa and B. napus. The potential role of some candidate genes in regulating these traits is discussed. Introduction Winter survival is an important characteristic for overwintering herbaceous crops, such as oilseed Brassica, and depends on the expression of many in- teracting traits. One of these traits is freezing toler- ance, and this can be increased in some genotypes by acclimating plants to cold temperatures (Kacperska- Palacz 1978; Palta 1992). Acclimated and non-accli- mated freezing tolerances were found to have sepa- rate genetic control in an interspecific cross of diploid potato species (Stone et al. 1993), and different puta- tive quantitative trait loci (QTL) were identified for these two traits in oilseed B. rapa (Teutonico et al. 1995). Flowering habit also can be related to winter survival, especially in crop species having variation in flowering response to vernalization. In barley (Hayes et al. 1993a, 1993b; Pan et al. 1994) and wheat (Galiba et al. 1995; Storlie et al. 1998), a ho- mologous chromosome region in each species was found to affect both vernalization responsive flower- ing time and cold hardiness. Oilseed B. rapa and B. napus include both annual and biennial types, the latter of which require vernal- ization to flower and are grown as an over-wintering crop in northern climates. Biennial forms generally have a higher frequency of winter survival; and win- ter survival was correlated with acclimated freezing tolerance in a study including annual and biennial cultivars (Teutonico et al. 1993). We previously ana- 201 Molecular Breeding 9: 201–210, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.