Gender variation
INTRODUCTION
As we have seen in previous Sections the reproductive biology of sycamore is complex and therefore can not be easily categorized and this appears to be a common feature of maples. For instance in Acer rubrum, categorized as being "polygamodioecious" (a population consists of male plants, female plants, and individuals with both types of flowers or hermaphrodites), Primack & McCall (1986), using a measure of gender devised by Lloyd (1980), found that 85% of the individuals in a population could be classified as male or females, the remaining being either predominantly female or male. This quantitative method has rarely been used to assess the functional gender of forest trees. In order to gain a better understanding of the floral reproductive biology of sycamore a population was investigated for gender variation.
METHODS
In May and June 1986, the gender variation of 21 trees was investigated at Murlough Bay, Co. Antrim (IGR D1942). The trees were visited on a 4-6 day basis depending on weather conditions. On each individual a total of six inflorescences were selected from three branchlets at height between 1.5m and 2.5m above the ground. On trees which exhibited clear differences between terminal and axillary shoots, three shoots of each were included. On each inflorescence, flowers at anthesis or just prior to anthesis were counted, their position on the inflorescence recorded, and were collected for the investigation of biomass allocation (not included here).
Functional gender can be estimated following the method devised by Lloyd (1980). It is a measure of the potential of a plant contributing genes to the next sexual generation by counts of functional androecial and gynoecial units. Gender is then estimated using the following equation:
- G = f /[ f +( m * E )]
G denotes functional femaleness varying between 0 and 1, f is the number of functionally female flowers, and m is the number of functionally male flowers. An equivalence factor (E) equates male and female fitness for the population. It is the total number of functionally female flowers in the population divided by the total number of functionally male flowers in the population. In this case a subset of the flower population was used, i.e. three terminal and three lateral inflorescences from each tree.
Fruit set was also investigated. On each tree 12 inflorescences were tagged and the number of female flowers counted. In late September the number of fruits on the infructescences was recorded. Data are available for 18 trees.
RESULTS
The total number of male and female flowers in the population was obtained by summing the values for the individual trees. In 1986 six inflorescences on 21 trees at Murlough Bay carried 6888 male flowers and 1076 female flowers. The equivalence factor (E) had a value of 0.1562, that is roughly there were 6.5 male flowers for one female flower.
Figure 1. Distribution of gender in a sycamore population in 1986.
Femaleness (G) of each individual is plotted against the rank of the
tree within the population for this character (
= protandrous individuals,
= protogynous individuals).
The distribution of gender among the population is shown in Fig. 1. Mixed sexuality was observed in all investigated individuals with about half of the trees exhibiting a G value between 0.4 and 0.6. Fig. 1 shows that there is a tendency for protogynous individuals to be predominantly female while protandrous individuals are predominantly male. Two individuals, protandrous trees M30 and M10, are noticeable exceptions as they have a gender estimate of approximately 0.7. Both trees are protandrous and Mode D (see Sex expression) which consist of inflorescences with a flowering sequence of male flowers followed by female flowers and another sequence of male flowers. In other protandrous trees as the G value decreases, inflorescences become predominantly of Mode C (male-female) and in tree M21 (G=0.11) pure male inflorescences (Mode E) are found. Gender had no significant impact on the fruit set of protandrous or protogynous individuals (Fig. 2). Fruit set was generally lower and variation greater among protandrous trees than among protogynous individuals.
Figure 2. Fruit set in relation to gender in sycamore in 1986 at Murlough Bay
( = protandrous individuals,
= protogynous individuals).
DISCUSSION
The Murlough Bay sycamore population consists predominantly of individuals of mixed sexuality with a gender value around 0.5, despite the observed sexual dimorphism. This is in contrast with the estimates of gender obtained in A. rubrum (the tree population was 70% male, 15% female and 15% of mixed sexuality) by Primack & McCall (1986) and gender estimates of ash in Ireland. Ash (Fraxinus excelsior) is a species whose sexuality is as poorly reported as that of sycamore and is variously described as polygamous, trioecious or, subdioecious (Binggeli & Power 1991). Gender estimates of ash indicate that trees were predominantly male and female (respectively 38% and 27%) and the remaining were of mixed sexuality (Fig. 3). Although gender estimates indicate that ash is closer to dioecy than sycamore, ash still bears functionally hermaphrodite flowers. Other differences include a large difference in E values, 0.156 for sycamore and 0.441 for ash.
Figure 3. Distribution of gender in a natural population of ash (Fraxinus
excelsior)
in 1989. Femaleness (G) of each individual is plotted against the rank
of the tree within the population for this character (Binggeli &
Power unpublished).
Primack & McCall (1986) pointed out that it was important to check for fruit production in investigations of plant gender. In their study they found that some inconstant males some years produce numerous female flowers but these flowers rarely develop fruits. Although in sycamore there is a difference in fruit set between the sexual morphs no difference was observed in relation to gender. However it is not possible to explain the outlying points observed in Fig. 2. It is probable that some of the estimates of fruit set do not fully represent good estimates of seed set as it has since been observed that the proportion of empty fruits is higher in some protandrous individuals.
As pointed out in sex expression section, yearly variation in sex expression caused by environmental factors are common. Changes in gender have been observed for instance in A. rubrum by Primack & McCall (1986), who suggest that estimates of gender should be gathered over a period of several years. A longer study of gender variation in sycamore might explain some of the variation reported here.