Barriers to sexual reproduction in Polygonum viviparum: a comparative developmental analysis of P. viviparum and P. bistortoides

Abstract

Polygonum viviparum is widely distributed in arctic and alpine regions of the northern hemisphere. Fruit set has never been observed in North American populations and has been reported only very rarely in Europe. Although this species is extremely well studied, the impediments to successful fruit production are unknown. We investigated the sexual reproductive process in P. viviparum growing in the southern Colorado Rocky Mountains. For comparison, we also examined this process in the sympatric congener P. bistortoides, in which reproduction is exclusively sexual. Lack of viable fruit production in P. viviparum has no single developmental explanation; defects occur in each of the processes and structures associated with sexual reproduction studied, yet, these processes and structures also appear to function normally in at least some flowers or individuals. Development is abnormal in many ovules of P. viviparum, however, comparison with P. bistortoides shows that these abnormalities do not contribute to differences in seed production between the two species. The virtual absence of sexual reproduction in P. viviparum appears to be due largely to a low rate of fertilization and to embryo/fruit abortion.

Fig. 1. Early ovule development in P. bistortoides (A, C and E) and P. viviparum (B, D and F). A, B, Nucellus surrounded by ovary wall. A megasporocyte is evident in B. C, D, Megasporocyte(s) entering prophase of meiosis I. The inner integument has been initiated. E, F, Megasporocyte(s) in meiosis, the outer integument has been initiated. B, Nucellar beak; ii, inner integument; M, megasporocyte; N, nucellus; O, ovary wall; oi, outer integument. Bars = 25 µm.

Fig. 2. Megaspore formation in P. bistortoides (A, C) and P. viviparum (B, D). A, End of meiosis II. The four spores that will form a tetrad are evident but the last walls have not yet formed. B, Linear tetrad. The chalazal‐most spore is on an adjacent section; its position is indicated by the arrow. C, D, The chalazal‐most spore (arrows) of each tetrad is functional, the remaining spores degenerate. A hypostase begins to differentiate at the base of the nucellus. Note multiple megaspores in P. bistortoides (C). ii, Inner integument; oi, outer integument; H, hypostase. Bars = 25 µm.

Fig. 3. Mature ovules of P. bistortoides (A and B) and P. viviparum (C). Part B shows the section adjacent to that in part A showing the position of the fusion nucleus (arrow) relative to the egg. C, Arrow indicates position of the fusion nucleus. E, Egg; ii, inner integuments; N, nucellus; oi, outer integuments; S, synergids. Bars = 50 µm.

Fig. 4. Abnormal ovules of P. bistortoides (A, C and E) and P. viviparum (B, D and F). A and B, Ovule with both outer and inner integuments but no evidence of megasporocyte or megaspore formation. Compare with Fig. 2. Bars = 25 µm. C and D, Ovules appear nearly mature in anatomy yet have collapsed (A) or are incomplete (B) gametophytes. Part D shows a small gametophyte with synergids and an egg cell. Bars = 25 µm. E, F, Mature ovules with solid nucellus and collapsed gametophytes. The ovules are similar in size to those containing normal female gametophytes. The collapsed cells in the centre of the nucellus indicate that a gametophyte began, but did not complete, development. Bars = 50 µm. G, Gametophyte; ii, inner integument; N, nucellus; oi, outer integument.

Fig. 5. Pollen viability as assessed by the FCR reaction.