Seed Siring Success Research Articles

Prezygotic mate selection is only partially correlated with the expression of NaS-like RNases and affects offspring phenotypes.

Nicotiana attenuata styles preferentially select pollen from among accessions with corresponding expression patterns of NaS-like-RNases (SLRs), and the postpollination ethylene burst (PPEB) is an accurate predictor of seed siring success. However, the ecological consequences of mate selection, its effect on the progeny, and the role of SLRs in the control of ethylene signaling remain unknown. We explored the link between the magnitude of the ethylene burst and expression of the SLRs in a set of recombinant inbred lines (RILs), dissected the genetic underpinnings of mate selection through genome-wide association study (GWAS), and examined its outcome for phenotypes in the next generation. We found that high levels of PPEB are associated with the absence of SLR2 in most of the tested RILs. We identified candidate genes potentially involved in the control of mate selection and showed that pollination of maternal genotypes with their favored pollen donors produces offspring with longer roots. When the maternal genotypes are only able to select against nonfavored pollen donors, the selection for such positive traits is abolished. We conclude that plants' ability of mate choice contributes to measurable changes in progeny phenotypes and is thus likely a target of selection.

Investigation of pollen competition between wild and cultivated sorghums (Sorghum bicolor (L.) Moench) using simple sequence repeats markers

In self-compatible plant species stigmata receive a mixture of self and outcrossed pollen and competition between them is expected to play a major role in determining the pollen-mediated gene flow. The use of male sterile bait plants in field trials to demonstrate the rate of gene flow is questionable due to lack of pollination competition. However, little direct evidence has been published. A field experiment of male sterile and male fertile sorghum pollen recipient bait plants was conducted to evaluate pollen competition between wild and cultivated sorghums and the effects of pollen competition on gene flow assessment. Pollen competition between wild and cultivated sorghums was estimated from two-component pollen mixtures of wild and cultivated sorghum (1:1 ratio) applied to wild, cultivated and male-sterile maternal bait plants. Paternity was determined in the progeny using two diagnostic Simple Sequence Repeat markers. The study found that self pollen has higher seed-siring success. Maternal genotype influences the siring ability of the pollen donor components which significantly deviated from the 1:1 pollen loads. The study showed that published estimates of gene flow derived from studies using male-sterile bait plants seriously overestimate gene flow and that pollen competition may be a significant factor influencing outcrossing rates. The results suggest that the predominant direction of gene flow is from cultivated to wild sorghum, potentially leading to introgression of crop genes into wild sorghum. Pollen competition should be taken into account in gene flow estimation, since presence of self-pollen can account for over half of seed produced irrespective of maternal genotype.

The role of pollen in the changing environmental conditions of Scots pine

Rapid climate warming poses a challenge to forest trees adapted to cooler conditions. However, a genetic safety net may exist for wind pollinated tree species in the form of long distance gene flow mediated by pollen containing alleles that are (pre)adapted to a warmer climate. In order to examine long distance gene flow in Scots pine (Pinus sylvestris L.), we monitored flowering phenology in central, northern, and northernmost Finland and measured the amount and viability of airborne pollen over four years. Viable airborne pollen grains were detected during female flowering periods and prior to local pollen shedding in all study sites. Depending on the year and study site, female flowers were exposed only to pollen originating elsewhere for between one and four days. Foreign airborne pollen represented 2.3% of all viable grains detected and varied between years and study sites. Artificial pollination trials were conducted to evaluate the effect of pollen origin on seed siring success. Pollen genotypes originating from southern Finland sired 76% and 48% of seeds in competition experiments where pollen from both populations was introduced simultaneously into the female strobilus. Furthermore, we investigated the importance of pollen arrival order by introducing pollen of northern and southern genotypes in to the strobilus at two-hourly intervals. Northern genotypes sired 76% of the analysed seeds when injected first but when southern genotypes were supplied first, both pollen types enjoyed equal success. The first pollen grain in the pollen chamber did not always fertilize the ovum. Instead there is likely a more subtle and complex form of competition between genotypes and their pollen grains. To examine chemically-mediated pollen interactions we conducted an in vitro germination experiment where different genotypes were in chemical but not physical contact. Both positive and negative interactions were found. We found highly negative effects in the germinability of northern pollen grains when in contact with southern pollen, and an increase in the germinability of southern pollen when in contact with northern pollen. There was no variation in the size of dry pollen grains between sources, but after hydration and germination, northern pollen grains were larger than southern pollen. Genotypes with high hydration rates had low germinability and slow pollen tube growth rates. This suggests that early germination and growth of the pollen tube is more dependent on pollen storage materials and less dependent on water intake. Pollen source affected the outcome of competition experiments and therefore is an important factor in long distance gene flow. However, genotype also plays a role and must be taken into account.

Post-pollination mechanisms in Nicotiana longiflora and N. plumbaginifolia: pollen tube growth rate, offspring paternity and hybridization

In natural populations where interfertile species coexist, conspecific and heterospecific pollen can be delivered to the stigmas. Post-pollination mechanisms might determine the seed siring success of different pollen donors within species as well as the chances for hybridization between species. Nicotiana longiflora and N. plumbaginifolia occur in sympatry in Northwest Argentina, where they have overlapping flowering seasons and share floral visitors. We explored (1) pollen tube growth rates for outcross versus self pollen in single-donor pollinations; (2) siring success of self versus outcross pollen donors in competitive pollinations, and (3) possibilities for hybridization by performing two- (outcross conspecific vs. heterospecific) and three-pollen donor (self vs. outcross vs. heterospecific) crosses. In N. longiflora, both pollen tube growth rate and siring success favored outcross pollen over self pollen and strong rejection of heterospecific pollen. In N. plumbaginifolia, pollen tube growth rate was similar for self and outcross pollen, self pollen sired similar numbers of offspring than outcross pollen and heterospecific pollen sired roughly the same number of progeny than self pollen. Results suggest that in natural sympatric populations, interspecific crosses would likely lead to unidirectional hybridization with N. plumbaginifolia as the seed parent.

Southern pollen sired more seeds than northern pollen in southern seed orchards established with northern clones of Pinus sylvestris

Contamination by southern pollen is a considerable problem in Scots pine (Pinus sylvestris L.) seed orchards established with northern clones. This study investigated whether the contamination was due to the competitive superiority of southern pollen by carrying out competition trials using mixtures of pollen from northern and southern populations of Scots pine. Trials were performed in a southerly seed orchard established with clones originating from northern populations. Seed paternity (siring) was determined through the analysis of allozyme variation. Southern genotypes sired significantly more seeds (76%) than their northern competitors and across all mixed-pollen crosses. Maternal genotype had no effect on seed siring success. The mean flower abortion rate was lower in southern pure-pollen crosses and mixed-pollen crosses than in northern pure-pollen crosses. The results show that local pollen may induce high levels of background pollination in southern seed orchards composed of northern genotype grafts. These results must be taken into account when aiming to produce suitable reforestation material for northern areas.

How robust is nonrandom mating in wild radish: do small pollen loads coupled with more competing pollen donors lead to random mating?

In previous studies of the weedy annual Raphanus sativus we have demonstrated that mating is nonrandom in greenhouse plants, suggesting that sexual selection is possible. To investigate how these greenhouse results might translate to conditions more similar to the field, we manipulated both pollen load size and the number of competing pollen donors on stigmas. While the smallest pollen loads (22 grains per stigma) were small enough to reduce fruit and seed set, seed siring success was unaffected by pollen load size. When the number of competing donors in a mixed pollination was increased to four, the proportion of seeds sired by the pollen donors was the furthest from expectation, suggesting that nonrandom mating increases as the number of donors per pollination increases. There was no significant interaction between pollen load size and number of competitors per pollination. Overall, mating remained nonrandom across all treatments. Thus differential seed paternity is likely to occur in the field as well as in the greenhouse.

Males outcompete hermaphrodites for seed siring success in controlled crosses in the polygamous Fraxinus excelsior (Oleaceae)

Polygamy (including trioecy and subdioecy), the co-occurrence of males, hermaphrodites, and females in natural populations, is a rare and poorly studied breeding system expressed in Fraxinus excelsior L. (Oleaceae), a wind-pollinated tree. Here we investigate siring ability of pollen from male vs. hermaphrodite individuals to better understand this sex polymorphism. We conducted single-donor and two-donor pollination experiments and compared both fruit set and seed siring success, assessed with polymorphic microsatellite markers, of male and hermaphrodite individuals. Single pollen donor crosses allowed us to verify the male function of hermaphrodites. However, pollen from hermaphrodites was much less proficient than male pollen, with males siring 10 times as many fruits in single donor pollination treatments. This result was strengthened by the surprisingly low reproductive success of hermaphrodites in pollen competition conditions: of the 110 seedlings analyzed three were selfed and only one was sired by the hermaphrodite donor. The remaining 106 were sired by the male pollen donor. These results raise the question of the maintenance of male fertility in hermaphrodites in Fraxinus excelsior. Male function of hermaphrodites in this species now needs to be assessed under field conditions.

Patterns and mechanisms of selection on a family-diagnostic trait: evidence from experimental manipulation and lifetime fitness selection gradients.

Plant traits that show little variation across higher taxa are often used as diagnostic traits, but the reason for the stasis of such traits remains unclear. Wild radish, Raphanus raphanistrum, exhibits tetradynamous stamens (four long and two short, producing a dimorphism in anther height within each flower), as do the vast majority of the more than 3,000 species in the Brassicaceae. Here we examine the hypothesis that selection maintains the stasis of dimorphic anther height by investigating the effects of this trait on pollen removal, seed siring success, and seed set in R. raphanistrum using both experimental and observational methods. Observational selection gradient analysis based on lifetime seed siring success provided evidence for an optimum dimorphism that was greater than zero in one of three years. In both experimentally manipulated and unmanipulated flowers, more pollen was removed in single visits from flowers with less dimorphism. There was no significant effect of anther dimorphism on female fitness (seed set). Therefore, there is some evidence to suggest that selection is maintaining anther dimorphism in wild radish, and that higher male fitness might result from restriction of single-visit pollen removal. We discuss these results in light of pollen presentation theory.

PATTERNS AND MECHANISMS OF SELECTION ON A FAMILY-DIAGNOSTIC TRAIT: EVIDENCE FROM EXPERIMENTAL MANIPULATION AND LIFETIME FITNESS SELECTION GRADIENTS

Plant traits that show little variation across higher taxa are often used as diagnostic traits, but the reason for the stasis of such traits remains unclear. Wild radish, Raphanus raphanistrum, exhibits tetradynamous stamens (four long and two short, producing a dimorphism in anther height within each flower), as do the vast majority of the more than 3000 species in the Brassicaceae. Here we examine the hypothesis that selection maintains the stasis of dimorphic anther height by investigating the effects of this trait on pollen removal, seed siring success, and seed set in R. raphanistrum using both experimental and observational methods. Observational selection gradient analysis based on lifetime seed siring success provided evidence for an optimum dimorphism that was greater than zero in one of three years. In both experimentally manipulated and unmanipulated flowers, more pollen was removed in single visits from flowers with less dimorphism. There was no significant effect of anther dimorphism on female fitness (seed set). Therefore, there is some evidence to suggest that selection is maintaining anther dimorphism in wild radish, and that higher male fitness might result from restriction of single-visit pollen removal. We discuss these results in light of pollen presentation theory.

Seed paternity in flowering plants: an evolutionary perspective

The ultimate importance of paternal contributions to fitness and of post-pollination selection in flowering plants have remained elusive, largely because of the technical difficulty of assigning paternity. I review empirical studies that use heritable markers to investigate per-fruit seed paternity in natural populations and after experimental multiple-donor pollination. Thirty-one studies covering 23 species from 16 plant families document that in natural populations seeds from a single fruit are often fathered by multiple pollen donors (5 species from 5 families), that donors can differ significantly in seed-siring success (8 species from 6 families), that variation in pollen tube growth rates can be heritable (n = 1 out of 4 studies), that donor and recipient genotypes can simultaneously affect paternity (n = 2), and that temporal order of pollen deposition (n = 1) and environmental effects(n = 2) affect the outcome of pollen competition. These studies also investigate the role of post-pollination selection in the avoidance of inbreeding and for species boundaries. Most studies of male reproductive success in plants to date base on isozyme electrophoresis. The availability in the last decade of highly polymorphic molecular markers such as microsatellite DNA has been expected to open new possibilities to investigate competition and selection during the gametophytic phase. Yet, to date, there is still need for greater data wealth on seed paternity to test theories of sex allocation and to gain deeper understanding of floral trait evolution and of the evolutionary consequences of post-pollination selection in flowering plants.

Genotype-environment interactions in pollen competitive ability in an anemophilous tree, Betula pendula Roth.

This study describes genotype-environment interactions in pollen competitive ability expressed as pollen-tube growth rate and seed-siring success in Betula pendula Roth. A factorial crossing design was applied using the same maternal and paternal clones in two different environmental conditions, in a B. pendula seed orchard established in a greenhouse and at an outdoor clone collection. Both single donor and two-donor pollinations were employed. Female inflorescences were collected after a fixed time of germination, pollen-tube lengths were measured for each cross, and paternity of the seeds sired by two-donor pollen mixtures was analyzed using isozyme markers. The pollination site had a significant influence on pollen-tube growth rate and seed-siring success. Significant interactions between pollination site and pollen donor indicated genotype-environment interactions in pollen-tube growth rate and seed-siring success. A highly significant positive correlation between pollen-tube growth rate and seed-siring success was found in the greenhouse but not at the outdoor clone collection. These results suggest that the pollen-tube growth rate can be a predictor of seed-siring success in controlled greenhouse conditions, where differences among maternal plants are mainly of genetic origin, but not in more heterogeneous outdoor conditions. In natural birch stands, environmental maternal effects probably diminish the significance of pollen competition for sexual selection in Betula pendula. At seed orchards, the effects of environmental conditions on pollen competitive ability can have important consequences for the genetic composition of the seed crop.

Pollen competition and seed-siring success in Picea abies.

The aim of the present work was to study pollen-tube competition in Picea abies. Controlled crossings were performed with pollen mixtures including pairs of pollen lots with fast and slowly elongating pollen-tubes. Paternity analysis using isozyme markers was performed on the progenies in order to study whether the in vitro pollen-germination vigour corresponds to the proportion of seeds sired by the pollen donor. Paternal success was found to be unequal, 15 out of 23 crossings producing progeny that differed significantly from the hypothetical ratio of 1:1. The paternal contribution in the majority of the crossings was as expected: the pollen parent with more-vigorous in vitro germination sired more seeds than the less-vigorous pollen. In the case of two pollen mixtures, however, the seed-siring success summed over the maternal trees was the opposite to the expected value. Despite these aberrations, the results support the hypothesis that pollen-tube competition is one of the factors contributing to male fitness in P. abies.However, when all the other factors affecting pollination and seed set under natural conditions are taken into account, it is clear that the seed-siring success of a particular paternal genotype cannot be predicted reliably by measuring only the in vitro pollen vigour.