AN ULTRASTRUCTURAL PACHYTENE KARYOTYPE FOR MELAMPSORA-LINI

An ultrastructural karyotype was derived from five serially sectioned pachytene nuclei of two isolates of Melampsora lini. Each of the five reconstructed nuclei contained 18 distinct bivalents, which terminated at both ends on the nuclear envelope and represented 36 chromosomes homologously paired as synaptonemal complexes. The 18 bivalents formed a finely graded series of lengths ranging from 3.2 to 9.0% of the total length of all bivalents. No centromeres were resolved, precluding the use of the centromeric index as an aid in classification of bivalents. The only bivalent that could be identified among the five reconstructed nuclei was the one in terminal association with the nucleolus. Because no morphological changes accompany the progression of meiotic prophase I stages in the rust teliospore, nuclei at or near mid-pachynema were identified by using epifluorescence microscopy of fixed teliospore protoplasts stained with 4,6-diamidino-2-phenylindole (DAPI) from which walls were mechanically removed. The M. lini karyotype of n = 18 is significantly larger than that previously reported in the literature and agrees with the high number of virulence genes reported to segregate independently.