We have performed a large experiment with the goal of adding markers to the genetic map in the regions adjacent to the centromeres of linkage groups II and V. For the sake of clarity in this manuscript we will use chromosome numbers paralleling the linkage group numbers as published in the most recent version of the pea genetic map (4). It is well known (1-3) that the centromeric regions are in the vicinity of T-points of Hammarlund translocation, involving chromosomes 2 and 5. We decided to treat the heterozygotes for this translocation by EMS and then identify new mutations linked to T-points and to the markers situated in the centromeric regions of chromosomes 2 and 5 of the normal karyotype. The plant material used was hybrid stock designated ‘Tau-2.’ This hybrid was generated from the line WL1476 [translocated karyotype (K)] with genotype Cri, Gp, A, His(2-6)2121 (1), and a partially inbred line with normal karyotype (N) and genotype cri, gp, a, His(2-6)1123. The latter stock had a complicated pedigree, being in general derived from the lines WL1238, WL1018, our line SG, and a mutation cri induced in our laboratory with EMS in line SG.

In the hybrid Tau-2 (karyotype N/K) a cross structure is resolved in meiosis I, shown on Fig. 1. The hyperploid gametes carry the additional minor interchange chromosome 52, composed of the short arms of the chromosomes 2 and 5. After combining with euploid gametes, viable and fertile trisomics are formed (1-3).

Selfing Tau-2 results in five main classes of progeny: structural homozygotes N/N for the normal and interchange (K/K) karyotype, structural heterozygotes (N/K), and tertiary trisomics with karyotypes N/N+52 and N/K + 52. The recessive alleles cri, gp, and a are located near the T-point of chromosomes of normal karyotype, while the dominant alleles reside on the interchange chromosomes. Therefore we can easily distinguish structural homozygotes N/N (with the phenotype cri gp a) and trisomics N/N+52 (phenotype Cri gp a). Trisomics (Cri gp a) can be confused with the very rare cross-overs between the breakpoint and cri, but they can be easily recognized by slightly reduced pollen fertility, large bracts (3), and by examination of their progeny. Class N/K (with the phenotype Cri Gp A) can be distinguished from K/K by pollen semisterility and codominant expression of histone H1 subtypes (1123+2121) encoded by the gene cluster His(2-6). This same phenotype can be exhibited by another class of trisomics N/K+52. However, this last genotype can be recognized by a weak growth habit and large bracts.

Among 800 plants produced from selfing the structural heterozygote, Tau-2, we obtained 44 trisomics (N/N+52). In addition we identified seven cross-over events between the T-point and gp, three between the T-point and a, none between the T-point and cri or between T-point and His(2-6).

We imbibed 2165 F4 seeds from Tau-2 in 0.15% solution of EMS overnight and grew the resulting plants hydroponically in the greenhouse. Among the M1 plants only structural heterozygotes were selected and their progeny germinated. Similarly, among M2 plants only structural heterozygotes (one M2 plant of each family) were allowed to produce seeds. By this procedure we eliminated about half of the mutations not linked to the T-points. The analysis of new mutations was carried out on M3 families grown in the field. We paid particular attention to families in which visible alterations of phenotype co-segregated or were negatively correlated with markers cri and a. Several mutations in the vicinity of the T-points were obtained. Here we report the results for one of them, a chlorophyll mutant of the xantha type.

Among the M3 families we found one in which 9 of 10 plants with phenotype cri had no chlorophyll but, judging by their yellow color, normal level of carotinoids. Another 34 plants of this family were normal green (Table 1). This new chlorophyll mutation was called golden (gld). All the nine gld plants died after producing 5 to 6 nodes but no flowers; however, absence of an anthocyanin ring at the base of the stipules indicated the phenotype a. In addition, all the mutants had a His(2-6) haplotype 1123. Thus, all mutants clearly had a normal karyotype (N/N). The plants with phenotype Cri Gld exhibited three phenotypic classes with respect to other markers (Table 1). Three, presumably trisomics, had white flowers with large bracts and the haplotype 1123. Two other classes had colored flowers with vexillary or no bracts. The heterozygotes (1123/2121) had a semisterile pollen (karyotype N/K), while the homozygotes (2121/2121) had normal pollen.