Ratio of protandrous to protogynous individuals and the inheritance of these characteristics
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A number of tree species are dioecious, but few have a 1:1 ratio of male to female individuals and often the sex ratio is variable, for example within a genus as in Salix (Elmqvist et al. 1988) or between populations of the same species (e.g. Populus tremula, Valentine 1975). The genetics of sex determination in plants is poorly known and a few cases of heteromorphic chromosomes have been established in tree species (e.g. Gingko biloba, Ilex serrata) or the evidence is inconclusive as is the case in Acer negundo, Populus and Salix species with reports of both the presence and absence of sex chromosomes (Chattopadhyay & Sharma 1991). The genetics governing mating types in Juglans regia has been investigated by Gleeson (1982). His controlled crosses indicated that the mode of heterodichogamy is regulated by a dominant recessive, Mendelian factor of two alleles at a simple locus, with protogyny as the dominant type.
As described in the Sex Expression Section sycamore exhibits a sexual dimorphism. In this heterodichogamous species a tree is protandrous when its inflorescences start with a sequence of functionally male flowers followed by a sequence of functionally female flowers, or protogynous when the reverse sequence occurs. Recently, in a study of the floral biology of sycamore Pigott & Warr (1989, p 99) have claimed that: "In most trees the male flowers normally open before the female", suggesting that most trees are protandrous. A similar claim is made by Klaehn (1958) for both sycamore and Norway maple.
In this Section, the ratio of protandrous to protogynous individuals in sycamore in Ireland and Scotland is reported and compared to previously published accounts. Preliminary results on the mode of inheritance of heterodichogamy in sycamore are given.
Samples were obtained from sycamore populations containing at least 15 trees. In each population most of the individuals were clearly planted and in many cases are likely to have been half-sibs, but occasionally naturally regenerated trees were included. The survey was carried out in nine Southern Irish populations in 1984; eight Northern Irish populations between 1984 and 1986 and eight Scottish populations in 1989. The sexual morph was determined using the method described in Binggeli (1990).
In order to obtain information on the genetics of heterodichogamy in sycamore, seeds from two protogynous trees (M7 and M9) were collected from Murlough Bay in 1985. The seeds were planted indoors in early 1986 and later planted out in Switzerland in plastic containers. The trees were irregularly watered and given fertilizer. Since 1988 the terminal buds were removed early in the season to reduce vegetative growth and to induce flowering. By 1991 19 trees had flowered.
Variation in the ratio of protandrous to protogynous individuals
As shown in Table 1, a large variation in the ratio of protandrous to protogynous individuals between populations was observed but few of the ratios were significantly different from 1:1 and both sexual morphs were represented at every site. In each survey more protogynous individuals were recorded with 51.7%, 52.9% and 57.4% in both Irish surveys and Scotland respectively, but the differences were not significant. Only four out of the 27 populations studied had a significant difference between the number of protandrous and protogynous individuals and in all instances the bias was towards protogyny.
Table 1. Number of protandrous and protogynous trees and their ratios in sycamore populations in Ireland and Scotland. IGR = Irish Grid Reference, BGR = British Grid Reference; Pa = protandrous & Po = protogynous individuals, Sign. level = significance level, NS = not significant at the 0.05 level.
Inheritance of protandry and protogyny
The progeny of tree M9 consisted predominantly of protogynous individuals (78.6%, N=14), whereas that of M7 is purely protandrous but this result is based on only five flowering individuals (Table 2).
Table 2. Number of protandrous and protogynous individuals among two half-sib progenies. Sign. level = significance level.
Table 3 shows that the proportion of protandrous and protogynous individuals varies widely between studies, including significantly biased ratios towards both protogyny and protandry. In the British Isles no significant differences were observed. Pigott & Warr (1989) appear to have based their statement, that most sycamores are protandrous, on de Jong's (1976) data, although his study showed a significant difference in the ratio of protogynous to protandrous individuals, with 57.1% of the trees being protandrous (Table 3).
Table 3. Ratio of protandrous to protogynous trees in sycamore. Pa = protandrous, Po = protogynous, Sign. level = significance level, NS = not significant at the 0.05 level, NS = not significant.
None of the studies reported the sex expression of natural populations and such studies are needed, but will be difficult to undertake because of the irregular flowering of sycamore in its natural range.
In other heterodichogamous species, such as Graya brandegei and Juglans hindsii, non-significant biases towards protogyny have been recorded (Gleeson 1982, Pendleton et al. 1988). However, in these species there is less variation between mating types among populations than in sycamore.
The observed ratio of protandrous to protogynous progenies of two protogynous trees is very different, but one set of results is based on a small sample size and many trees have yet to flower. In view of the wide variation in mating types observed among populations in Ireland and Scotland, but of an overall ratio close to 1:1, it is likely that the ratio among progenies is equal. A mode of inheritance similar to that suggested for Juglans regia by Gleeson (1982) is possible, but to be demonstrated, more data are required. These should be available in the future from the experiment already underway in Switzerland.
|Copyright © 1999 Pierre Binggeli. All rights reserved.|