Investigating the primary breeding systems of Asphodelus species

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In plants, the primary breeding systems are outbreeding, self-fertilization and apoximis. Outbreeding is defined as crossing between different individuals. Self-fertilization occurs when pollen fertilizes an ovule on the same plant. In flowering plants, inbreeding can occur through self-pollination within flowers or self-pollination between flowers on the same plant. Pollination can be done by either abiotic or biotic. Abiotic pollination refers to pollination that is mediated without the involvement of other organisms, for example, wind pollination. However, most plants use biotic pollination which requires pollinators such as birds, bees and butterflies. In this project, we are going to look at the breeding systems in Asphodelus and the foraging pattern of their pollinators.

In the genus of Asphodelus, two species- A. aestivus and A. fistulosus- are found in Mallorca. Both of them are not endemic to the island. The habitats of A. aestivus are dry grassland, garrigues and rocky or sandy ground. It is a tall plant, around 1 m in height. Leaves are flat with keeled central vein. The stamens are 14-19 mm long and the arrangement is more spread out, thus suggesting outbreeding. However, it has 3 or 4 open flowers per inflorescence, which suggests inbreeding since there is a higher probability for pollinators to visit the flower next to the pollinated flower.

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A. fistulosus is less common and grows in fields, garrigues and slightly wetter places. It is a shorter plant, around 25-50 cm. Leaves are semi-cylindrical with hollow center which make it easily distinguished from A. aestivus. The stamens are 6-7 mm long and the arrangement is slightly compact, thus suggesting inbreeding. However, it has 1 or 2 open flowers per inflorescence, which suggests outbreeding since there is a higher probability for pollinators to visit other plant due to fewer open flowers per plant.

The breeding system of every flowering plant species has a percentage of inbreeding to outbreeding, whether it favors towards inbreeding or towards outbreeding. The aim of this project is to identify the breeding behavior of both Asphodelus species. Two hypotheses were proposed. The first hypothesis is the population density affects inbreeding, and the second hypothesis is the number of open flowers per plant affects inbreeding. This project was carried out at Boquer Valley. The first hypothesis was only done for A. aestivus, as there were only a few bee pollinators for small density of A. fistulosus, and not enough observations can be done to collect sufficient data. Flowering plants of Asphodelus species are very attractive for pollinators due to the large number of showy flowers opened per day. The pollinators of both Asphodelus species are bees. Not all insects seen visiting flowers are pollinators. There are a few insect visitors. For A. aestivus, the bees foraged for pollen during the morning at 10 am till 2 pm in the afternoon, while pollinators of A. fistulosus foraged slightly later, from 11 am to 2 pm. There are higher foraging activities in the morning, and decreases in the afternoon.

Methods

This project was carried out for two days. We made observations on A. aestivus on day 1 to identify the breeding behavior and the hypothesis. Observations on A. fistulosus were done in day 2. We started to examine at 10 am in the morning when the bee pollinators are most active, and ended at 2pm when they are less active. We studied the natural populations of both Asphodelus species in an area near the entrance car park of Boquer valley.

On day 1, the breeding behavior of A. aestivus was determined by observing the pattern of flower visits by bee pollinators. We observed a population of A. aestivus in an area of 5 m x 5 m. When a bee visits flowers within a plant, it is considered as inbreeding. When a bee visits flowers from one plant to another plant, it is considered as outbreeding. The sequence of flower visits done by each bee pollinators were recorded down, for example in Fig 3, the sequence is I I O I O.

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To test the first hypothesis, we observed an area of high density of A. aestivus and an area with low density population. The high density area is filled with A. aestivus which are distributed very densely and compact, and we are not able to walk through the population. The low density area is filled with A. aestivus which are distributed less compact, and we are able to walk through the population. Both area were 5 m x 5 m. We used the same method in Fig. 3 to see the pattern of flower visits.

In order to test the second hypothesis, the number of within plant pollinations per A. aestivus was counted when a bee visited the flowers in a plant. We then counted the number of open flowers in the plant. For example in Fig. 4, there are 3 inbreeding events in a plant with 6 flowers.

On day 2, the breeding behavior of A. fistulosus was determined. There were fewer populations of A. fistulosus found in Boquer Valley as compared with A. aestivus. Furthermore, there was only a few numbers of pollinators, so instead of examining an area of 5 m x 5 m, we looked around the whole area of A. fistulosus population. The same method was used to observe the pattern of flower visits by bee pollinators (Fig.3). To test the second hypothesis, the method in Fig. 4 was used to observe the number of within plant pollinations per A. fistulosus. The number of open flowers per A. fistulosus was counted.

Furthermore, we also made observations to determine the number of successful pollinations. We counted the number of fruits per plant and the number of pedicels without fruits attached. Pedicels without fruits may be due to loss of fruits, immature fruit or failure of fruit production.

Results

Data for A. aestivus

Total number of inbreeding and outbreeding: 1201

Total number of inbreeding: 852

% of inbreeding: 70.9 %

% of outbreeding: 29.1 %

Data for A. fistulosus

Total number of inbreeding and outbreeding: 254

Total number of inbreeding: 48

% of inbreeding: 18.9 %

% of outbreeding: 81.1 %

Table 1: Percentage of inbreeding and outbreeding of A. aestivus in high density and low density

Total number of pollination events

Total number of within plant pollination events

Total number of cross-pollination events

% of within plant pollination

% of outbreeding

A. aestivus (high density)

672

481

191

71.6 %

28.4 %

A. aestivus (low density)

529

371

158

70.1 %

29.9 %

Table 2: Percentage of inbreeding and outbreeding of A. aestivus and A. fistulosus

Total number of pollination events

Total number of within plant pollination

Total number of cross-pollination events

% of within plant pollination

% of outbreeding

A. aestivus

1201

852

349

70.9 %

29.1 %

A. fistulosus

254

48

206

18.9 %

81.1 %

Table 3: Percentage of successful pollination of A. aestivus and A. fistulosus

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Total number of seed set for 6 plants

Total number of unsuccessful pollination for 6 plants

% of successful pollination

A. aestivus

720

147

83 %

A. fistulosus

303

138

68.7 %

Difference in proportions test

For percentage of within plant pollination in A. aestivus and A. fistulosus (Table 2),

=>The difference is statistically significant at a 95% level.

Correlation test results

Correlation value (r) for A. aestivus (Fig. 5) = 0.57

Correlation value (r) for A. fistulosus (Fig. 6) = 0.85

Pollinators and visitors

A. aestivus

Pollinators: Andrena species, honey bee and Bombus lucorum.

Visitors: Wasp, small black fly, flower scarab and green soldier beetle.

A. fistulosus

Pollinators: Andrena species and small black bee.

Visitors: small black fly.

90 % of the bee pollinators for A. aestivus are Andrena species. 10 % of them are honey bee and Bombus lucorum. The insect visitors are wasp, small black fly, flower scarab and green soldier beetle. For A. fistulosus, 90 % of the bee pollinators are Andrena species too, and 10 % of them are small black bee. According to Corbet (1990), Asphodelus species by attracting different pollinators by having a wide range of sugar concentration throughout the day to increase their pollination efficiency during the period of nectar presentation. During the day, the temperature and relative humidity affects the volume and sugar concentration. The progressive loss of water through evaporation results in an increase of sugar concentration. Kugler (1977) defined the morphology of the flowers as "large bee-dish shaped blossom" which allows the access of insects of a very wide range of size. Hymenoptera were the most frequent visitors.

Both Asphodelus species depends on pollinators for seed set. Even though not all insect visitors are pollinating the flowers, and not all the pollinators’ interactions result in successful pollination events, the high number of seed set in both Asphodelus species after fruit maturation shows a high percentage of successful pollination. According to Diaz Lifante (1996) a higher fruit-set and seed-set absolutely depends on insects for pollination. Even though Asphodelus are self-compatible, insect visits are required for effective pollination due to the position of the anthers and stigma which are far apart from each other. Diaz Lifante (1996) found there are higher fruit-sets and more developed seeds per fruit in cross-pollinated than in self-pollinated flowers. In addition, the female reproductive success of Asphodelus depends on both the amount and origin of the pollen supplied by their pollinators (Schuster et al., 1993).

Among flowering plants, it is commonly observed that a low proportion of the flowers successfully develop into fruits. It is no exception that Asphodelus shows low fruit-flower ratio. The limitation of seed production may be due to insufficient resources or due to lack of pollinators. Studies suggest that the low fecundity of outbreeding species may be due to their genetic load. Another explanation is the clogging of stigma due to interspecific pollen transfer. Based on Table 3, the percentage of successful pollination for A. aestivus is high (83 %), while A. fistulosus has slightly lower percentage (68.7 %). The limitation of seed set of A. fistulosus can be the result of either limited pollinator activity, which may limit total pollen supply to the stigma, or deficiency in the quality of the pollen. Pollen source is critical for Asphodelus which are self-compatible due to inbreeding depression. Schuster et al. (1993) refer the first mechanism of limitation as ‘quantitative pollen limitation’ and the second as ‘qualitative pollen limitation’. Pollen from the same plant may result in fewer mature seed. This could be due to an early expression of deleterious recessive alleles, which is also known as inbreeding depression. The advantage of fertilization by self pollen in Asphodelus is less obvious, but there could be a selective advantage to self-compatibility in Asphodelus if the negative effect of quantitative pollen limitation was stronger than the negative effect of self pollination throughout the lifetime of the plant (Schuster et al., 1993). This pollination limitation for Asphodelus seed is not a limiting factor throughout the lifetime. The authors also found a positive correlation between pollen amount and the number of total seeds per flower.

Based on Table 1, the percentage of within plant pollination of low density is slightly lower than that of high density, but both percentages are around 70%. This concludes that our first hypothesis is wrong; density does not affect inbreeding. However, this may also due to experimental errors. While doing our observations, the low density population is located right next to the high density population with no clear separation line or zone. This may affects the accuracy of the number of inbreeding events solely in low density population. Based on graph 1, the number of within plant pollination increases with the number of open flowers per A. aestivus. The r value (0.57) shows a moderate correlation between the two parameters. According to graph 2, the number of within plant pollination increases as the number of open flowers per A. fistulosus increases. The r value (0.85) shows a strong correlation between the two parameters. These results prove that our second hypothesis, that is the number of open flowers per plant affects inbreeding, is correct.

Based on the data analysis in Table 2, A. aestivus has a higher percentage of within plant pollination while A. fistulosus has a higher percentage of outbreeding events. This result shows that the breeding system of A. aestivus is mainly inbreeding, while A. fistulosus is mainly outbreeding. A “difference in proportions” test was done and the result proves that the breeding behavior of A. aestivus and A. fistulosus is significantly different. The average number of open flowers in A. aestivus is 17, while A. fistulosus has an average of 4 open flowers per plant. This relates to our hypothesis, as A. aestivus has more open flowers, it favors towards inbreeding, while A. fistulosus favors towards outbreeding due to its small number of open flowers per plant. The outbreeding breeding system of A. fistulosus maximizes heterozygosity thus promotes greater fitness or flexibility characteristics to respond and adapt to environmental change. However, outbreeding could be limited by pollinator preference for short distance visits. The benefit of having inbreeding breeding system increases homozygosity of A. aestivus. This maximizes the allele for a particular environment, so it is very well adapted. However, it may not adapt to changes in environmental conditions. Inbreeding reduces fertility of A. aestivus plants. The frequency of alleles being homozygous at a particular locus increases with the number of inbreeding events; hence inbreeding reduces the amount of variation in a population.

Inbreeding depression is a phenomenon in which inbreeding reduces the ability of a population to survive and reproduce. There are few traits such as pollen quantity, number of ovules, amount of seed, germination rate, growth rate and competitive ability shown to be subjects to inbreeding depression (Frankham et al., 2003). The interplay between several selective forces might be a possible reason for the evolution of inbreeding in A. aestivus, however, with inbreeding depression as the primary cost. The evolution of inbreeding may be due to inherent costs of outbreeding that will influence fitness of the parent plant (Solbrig, 1976).For instance, the direct costs of copious pollen production for effective fertilization take away from the potential for seed production (Jain, 1976). During sexual reproduction, parental fitness is reduced when only half of the genome is transmitted to the next generation, and this is known as the ‘meiotic cost’ (Solbrig, 1976). Another reason for the evolution of inbreeding is the fitness costs for offspring due to outbreeding. Outbreeding depression occurs when the co-adapted gene complexes that confer adaptation to a local environment are broken apart.

The evolution of inbreeding in A. aestivus may also be fostered by various other ecological phenomena.For example, low pollinator numbers lead to a reduction of seed set in insect cross-pollinated plants due to stochastic fluctuations in pollinator populations (Jain, 1976). Inbreeding thus benefits the populations that are subjected to environmental stochasticity during extreme weather conditions that hinder the transmission of pollens between plants (Lande and Schemske, 1985). Predominance of inbreeding occurs when outbreeding is selected against in the absence of inbreeding depression via partial dominance (Lande and Schemske, 1985). However, it is necessary for outbreeding to maintain genetic variation to ensure the long term survival of species (Frankham et al., 2003).

The effects of inbreeding in demography and persistence of natural population are often studied by researchers. Investigations on the population level of inbreeding have documented the effects in a number of species. Populations of an endemic prairie speciesSilene regiawere compared by Menges (1990), and results showed there was an increase in seed set with population size. In a related study, Oostermeijer et al. (1994) found that there was an increase in offspring fitness of Gentiana pneumonanthe with population size. In a small population of a related rare species, Gentianella germanica, a reduction in seed set causes a decrease in population. The occurrence of the reduction in plant fitness is independent of environmental variation, thus indicating that the inbreeding effects were the primary cause (Fischer and Matthies 1998). Heschel and Paige (1995 proposed that the effects of inbreeding depression may increase the susceptibility of populations to environmental stress.

Several field studies have provided evidence that support theoretical results about the evolutionary consequences of inbreeding. The variation of inbreeding effects with natural outbreeding rates was examined by Holtsford and Ellstrand (1990) in populations ofClarkia tembloriensis. The results obtained does not support the theory of populations with high natural rates of inbreeding will reduce inbreeding effects. All populations showed significant inbreeding depression for fecundity traits (Holtsford and Ellstrand 1990). Another similar study ofCollinsia heterophylla also shows that increasing level of natural inbreeding does not reduce inbreeding depression effects (Mayer et al. 1996).

A. aestivus is a hexaploid geophytes with 2n= 84 while A. fistulosus has two ploidy levels: diploid 2n= 28 and tetraploid 2n= 56 (Diaz Lifante, 1991). Several researchers have studied on the correlation between polyploidy, longlived cycles and asexual reproduction, and they proposed that outbreeding is a highly advantageous factor to stabilize polyploidy (Diaz Lifante, 1996). Outbreeding favours an increase of genetic variability, but leads to a reduction in seed production. This can be seen in Table 3 that the percentage of seed set in A. fistulosus is lower than that of A. aestivus. While doing our observations, we could not find sterile individuals of both Asphodelus species. The sterile individuals have reduced stamens with no fruits. Diaz Lifante (1996) proposed that A. aestivus have a high number of multivalent which are produced in meiosis due to its hexaploid condition. The chromosomal arrangements were observed in karyotypes.