Russian Wildrye [Psathyrostachys Juncea (Fisch.) Nevski] Richard R

_??_1990 by Cytologia,Tokyo Cytologia55: 639-643, 1990 Meiotic Associations at Metaphase l in Diploid , Triploid, and Tetraploid RussianWildrye [Psathyrostachys juncea (Fisch.)Nevski] RichardR.-C. Wang and JohnD. Berdahl USDA-ARS,Forage and RangeResearch Laboratory , Utah State University,Logan, UT 84322-6300and USDA-ARS, NorthernGreat PlainsResearch Laboratory, P. O.Box 459, Mandan,ND 58554,U. S. A. AcceptedJuly 6, 1990 Russian wildrye [Psathyrostachys juncea (Fisch.) Nevskil] is an important forage grass. It was introduced into the U. S.A. from the USSR and China in 1927 (Hanson 1972), but research and breeding programs were not initiated until the 1940's. Russian wildrye is a diploid (2n= 14) that was classified by Love (1984) as having the N genome. This grass is productive and resistant to environmental stresses but is difficult to establish due to poor seedling vigor. Because of its relatively low chromosome number, induced polyploidy may be an effective method to increase the size of cells and organs (such as the seeds), thus improving seedling emergence. Indeed, seedling emergence was significantly higher in autotetraploids induced by nitrous oxide (Berdahl et al. 1989) and colchicine (Lawrence et al. 1990) than in diploid con trols. This report describes the meiotic associations in diploid Russian wildrye and derived autotriploids and autotetraploids. Based on the results of this study, breeding strategies for Psathyrostachysjuncea are suggested. Materials and methods Tetraploid plants of this species were produced by nitrous oxide treatment following emas culation and pollination of selected plants from the cultivar 'Bozoisky' (PI 440627; developed in the USSR) and various other experimental strains (Berdahl and Barker, in preparation). Triploid plants were obtained through natural pollination of the tetraploids by diploids at Mandan, North Dakota, and through selfing of the diploid (utilizing the occurrence of an unreduced gamete) at Logan, Utah. Progenies of the tetraploids were grouped as triploid or tetraploid, depending on the pollen sources in the Mandan field nursery. Spikes of the plants were fixed in the Carnoy's solution (6:3:1 of ethanol: chloroform: acetic acid) for 24 to 48 hours before transferring into 70% ethanol for storage in a refrigerator. Meiotic analysis was carried out with pollen mother cells (PMCs) squashed and stained in aceto-carmine. Results and discussion Chromosome associations at the metaphase I(MI) in PMCs of the diploid averaged 1.74 rod bivalents +5.26 ring bivalents in 31 cells. This pattern was almost identical to that ob tained by Dewey and Hsiao (1983) with the exception that they observed a low frequency of univalents. The c values (mean arm-pairing frequency; Alonso and Kimber 1981) in the diploid plants were 0.876 and 0.869 in this study and that of Dewey and Hsiao (1983), re spectively. Seven triploid plants were isolated from the progenies of autotetraploids subjected to open pollination, presumably as a result of pollination by pollen grains from diploid plants grown in the same field nursery at Mandan, ND. The meiotic associations at MI in these seven tri 640 Richard R.-C. Wang and John D. Berdahl Cytologia 55 ploids were slightly variable (Table 1), reflecting the genotypic variations among the pollen sources as well as heterozygosity among the tetraploid maternal parents. Nevertheless, the mean pairing patterns of the seven triploids were similar to the one triploid produced by self ing the diploid plant in Logan, UT (Table 1). A maximum of seven trivalents was easily ob served (Fig. 1). The high trivalent frequencies observed in these triploids confirmed their autotriploidy and indicated the absence of a bivalentization system (Charpentier et al. 1988, Wang 1989, Wang and Hsiao 1989) in the parent selections. The mathematical analysis of Alonso and Kimber (1981) to estimate the c and x values for these triploids indicated their autotriploidy, because all the x values (relative affinity between the two closest genomes) were close to 0.5 (Table 1). However, the trivalent frequencies observed in these triploids were higher than expected based on their model. Closer examinations of the configurations of rod and ring biva lents in the diploid revealed that the seven chromosomes of the N genome contained two rod bivalents having either one or two chiasma(ta), two ring bivalents having two to three chiasmata, and three ring bivalents having three to four chiasmata at late pro phase (Fig. 2). The mean chiasma frequency per bivalent was 2.24 at MI. This is higher than the maximum of 2.00 assumed for the model of Alonso and Kimber (1981) for triploids. This violation of the basic as sumption of their model may explain the discrepency and may invalidate its use. Therefore, we used the equations of Jackson and Casey (1982), which assume from I to 4 chiasma(ta) per bivalent rather than the fixed Figs. 1-3. Meiotic pairing in triploid, diploid, 1 or 2 chiasma(ta) per bivalent in Alonso and tetraploid Psathyrostachys juncea, Russian and Kimber's model (1981), to calculate the wildrye. 1, a metaphase I PMC showing seven expected trivalent numbers for three subsets trivalents in the triploid. 2, a late zygotene cell showing two bivalents with 1-2 chiasma(ta) (arrow of chromosomes of the N genome. The heads), two bivalents with 2-3 chiasmata (small sums of trivalents from the subsets based on arrows), and three bivalents with 3-4 chiasmata different P values (0.75 to 0.95) ranged from (large arrows). 3, a metapase I PMC showing 4.52 to 5.74 (Table 2). These expected triva eight bivalents, two chain quadrivalents (small lent frequencies were close to those observed arrows), and one ring quadrivalent (large arrow) in the tetraploid. •~2,400. (Table 1). Jackson and Casey's models appeared to be more appropriate for the N genome chromosomes. The trivalent frequencies observed in these autotriploid plants of Psathyrostachys juncea (Table 1) are much higher than that reported for autotriploid Secale cereale (Niwa et al. 1989), in which 2.37 to 3.01 trivalents were observed. It appeared that chromosome length was not responsible for the variations in trivalent frequency. Both the N and R genomes have similar, long chromosomes (Hsiao et al. 1986), and the short chromosomes of the S and H genomes 1990 MeioticAssociations at MetaphaseI in RussianWildrye 641 formed 2.74 to 4.96 trivalents in their autotriploids (Wang 1990). The R-genome chromosomes could also form up to four chiasmata per bivalent, but the mean chiasmata per bivalent was 2.00 (Galindo and Jouve 1989). The difference in chiasma formation, which may be both quantitative (chiasma number) and qualitative (chiasma maintenance), might be responsible Table 1. Meanchromosome associations at metaphaseI of PMCsof autotriploid Psathyrostachysjuncea * c and x are mean arm-pairing frequency and relative affinity , respectively, according to Alonso and Kimber (1981). Table 2. Expected trivalent frequencies per PMC at different P values (Jackson and Casey 1982) for the three subsets of the N-genome chromsomes (x=7) Table 3. Mean chromosome associations at metaphase I of PMCs in autotetraploid Psathyrostachys juncea for the variations in trivalent frequency observed for different autotriploid species. Another possibility is that a partial bivalentization reduces the trivalent frequency in favor of the for mation of ring bivalents, thus shifting the pairing patterns toward the 2:1 model of Alonso and Kimber (1981) such as in the genus Thinopyrutn (Wang and Hsiao 1989). Both triploid and tetraploid Secale produced fewer trivalents and multivalents, respectively, than expected 642 RichardR. -C. Wangand John D. Berdahl Cytologia55 according to the equations of Wang (1989). This suggests the presence of a bivalentization system in the R genome. Conversely, the higher than expected trivalent frequency in the autotriploid Psathyrostachys, 5.08 vs. 3.76, could be attributed to the absence or inactivity of the bivalentization system in addition to the higher chiasmate formation per bivalent than the assumed maximum of 2.00. Meiotic associations of PMCs at MI in seven autotetraploid plants are presented in Table 3. Based on the diploid c value of 0.87 and the equation of Wang (1989), expected multivalent (trivalents plus quadrivalents) frequency in the autotetraploids was 2.47. A range of 2.07 to 2.69 and a mean of 2.43 multivalents were observed, thus confirming the expectation. How ever, our scores of quadrivalents (Fig. 3) in these plants might be too conservative in light of the trivalent frequency observed in triploids (Table 1). If the reasoning of an absence or inactivity of the bivalentization system in the triploid is accepted, we can assume that the select ed parents are lacking or heterozygous for the genes controlling bivalentization. Because bivalentization is governed by two or more recessive genes (Charpentier et al. 1988), the auto tetraploid cannot be bivalentized. Therefore, out observations of multivalent frequency in these autotetraploids could only be too conservative. Nevertheless, this level of multivalent frequency was comparable to that cited in Wang (1989) in the genus Agropyron, which has the P genomes in an autoploid series of diploid, tetraploid, and hexaploid species (Dewey 1984). Autopolyploidy did not affect fertility and seed production in crested wheatgrass (Agropyron spp.), despite of the lack of bivalentization. Therefore, autopolyploid Russian wildrye may not necessarily be less fertile than its diploid state. The mean pollen stainability in 15 auto tetraploid plants was 86% as compared to 87% in 14 diploid plants (Berdahl et al. 1989). It appears that bivalentization for autopolyploids or diploidization for allopolyploids is not so essential for high seed yield in outcrossing species as in self-pollinating species. Therefore, the same breeding strategies for crested wheatgrass (Asay 1986)can be applied for improvement of Russian wildrye. Both grasses are autopolyploid outcrossers having relatively high fertility in spite of high multivalent frequency.

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