Ratio of protandrous to protogynous individuals and the inheritance of these characteristics

INTRODUCTION

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.

METHODS

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.

RESULTS

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.

IRELAND 1984
Site	IGR	Pa	Po	Ratio	Chi2	Sign.level
Buncrana	C3432	24	20	1.20	0.72	NS
Donegal	G9287	8	11	0.73	0.47	NS
Ballyshannon	G8862	6	10	0.60	1.00	NS
Grange	G6548	4	12	0.33	4.00	0.05-0.01
Tobercurry	G5414	9	7	1.29	0.25	NS
Claremoris	M3475	7	8	0.88	0.06	NS
Kilcolgan	M4316	13	5	2.60	3.55	NS
Tralee	Q8416	11	10	1.10	0.05	NS
Killarney	V9697	6	11	0.55	1.47	NS
Total		88	94	0.94	0.20	NS
%		48.3	51.7
IRELAND 1984-1986
Site	IGR	Pa	Po	Ratio	Chi2	Sign.level
MurloughBay	D1942	11	16	0.69	0.93	NS
Ballycastle	D1142	13	23	0.57	2.78	NS
Ballyversall	C8935	10	11	0.91	1.10	NS
Coleraine	C8434	27	17	1.59	2.27	NS
Portstewart	C8237	8	21	0.38	5.83	0.05-0.01
Bellarina	C6732	14	16	0.88	1.33	NS
Artiferrall	D0222	19	17	1.12	0.11	NS
Derry	C1541	10	5	2.00	1.67	NS
Total		112	126	0.89	0.82	NS
%		47.1	52.9
SCOTLAND 1989
Site	BGR	Pa	Po	ratio	Chi2	Sign.level
Bettyhill	NC7263	3	12	0.25	5.40	0.05-0.01
Forres	NJ0458	13	9	1.44	0.73	NS
Archiestown	NJ2344	8	11	0.72	0.47	NS
Carronwest	NJ2342	6	10	0.60	1.00	NS
Carroneast	NJ2144	10	7	1.43	0.52	NS
Hermitage	NO0142	9	6	1.50	0.60	NS
Howgate	NT2657	5	11	0.45	2.25	NS
Leadburn	NT2455	4	12	0.33	4.00	0.05-0.01
Total		58	78	0.74	2.94	NS
%		42.6	57.4

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.

	protandrous	protogynous	Chi2	Sign. Level
M9	3	11	4.57	0.05-0.01
M7	5	0	5.00	0.05-0.01

DISCUSSION

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.

Country	Sample size	Year	% of Pa	% of Po	Chi2	Sign. level	Source
Germany	100	1958	62.0	38.0	5.76	0.05-0.01	Scholz 1960
Germany	70	1964	46.0	54.0	5.14	0.05-0.01	Semm 1965
Germany	116	1963	56.7	44.3	2.11	NS	Semm 1965
Netherlands	616	1967-70	57.1	42.9	12.02	<0.001	de Jong 1976
Ireland	182	1984	48.3	51.7	0.20	NS	This study
Ireland	238	1984-86	47.1	52.9	0.82	NS	This study
Scotland	136	1989	42.6	57.4	2.94	NS	This study

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.