Gametophytic selection is the selection of one

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

pollen grain over another through the means of

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

competition (see also certation), and that resulting

sporophytic A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyte develops from the zygote pr ...

generations are positively affected by this competition. Evidence for the positive effects of gametophytic selection on the sporophyte generation has been observed in several flowering plant species, but there are is still some debate as to the biological significance of gametophytic selection.

Non-random success of pollen tubes

Pollen competition hypothesis

The competitive ability of

pollen grains Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

( microgapmetophytes) is rooted in the expression of their haploid genomes. The haploid genes are expressed immediately after pollen development and during pollen

germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...

and pollen-tube growth. About 60% of genes expressed in the sporophyte are also expressed in the microgametophyte. This expression influences the ability of pollen tubes to compete during growth. When pollen competition occurs, the competitive ability is determined by differences between tube growth rate or the time it takes for germination to occur. Pollen completion is increased when pollen is not limiting and when pollen is in abundance relative to the number of ovules present in the ovary, but this does not guarantee pollen competition.

Non-random success of pollen

Studies on corn have observed a non-random success of pollen grains possessing different alleles resulting in ratios that differ than those expected by Mendel's Law of Segregation of Genes (certation). Pollen from a heterozygous sporophyte should exhibit an equal distribution of gametes inherited by offspring. Evidence of higher fertilization frequencies by pollen carrying one allele resulted in differences from expected random mating ratios.

Offspring quality

Evidence suggests that gametophytic selection may influence the fitness of seedlings in the next sporophytic generation. Studies on specific species have observed improvement of offspring quality suggesting that the rate of pollen-tube growth in the style is positively correlated with the rate of seedling growth in the next generation. In experiments, '' Dianthus chinensis'' demonstrated that when pollen tubes had to grow a longer distance through the style the offspring had increased vigor and competitive ability. Pollen competition is also one of the primary drivers for

cryptic self-incompatibility Cryptic self-incompatibility (CSI) is the botanical expression that's used to describe a weakened self-incompatibility (SI) system. CSI is one expression of a mixed mating system in flowering plants. Both SI and CSI are traits that increase the ...

favoring outcrossed pollen for fertilization.

Reduced inbreeding depression

Faster pollen tube growth rate in ''

Dalechampia ''Dalechampia'' is a genus of plant of the family Euphorbiaceae and of the monogeneric subtribe Dalechampiinae. It is widespread across lowland tropical areas (generally below 2,000 m ASL) primarily in the Americas with smaller numbers of species ...

scandens'' results in reduced

inbreeding depression Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. In ...

in mixed-mating systems due to intense pollen competition after self-pollination. Gametophytic selection was apparently responsible for increased seed mass and radicle growth in selfed seedlings.

Sex ratios

Experiments on ''

Rumex hastatulus ''Rumex hastatulus'', commonly known as the heartwing sorrel, is an annual flowering plant species in the family Polygonaceae otherwise known as the buckwheat family. It is found spread out through the eastern and southern United States. Descri ...

'' demonstrated that sex ratios differences were not induced by environmental or biotic variables, but that pollen competition did result in skewed sex ratios.

Seedless tracheophytes vs. angiosperms

Current hypotheses suggest that gametophytic selection in early seedless land plants would have seen negative repercussions due to the limitations imposed by environmental selection on independent gametophytes, like those of bryophytes and ferns.

Polyploidy Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei ( eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...

may have been a mechanism that avoided these repercussions in modern ferns. Flowering plants may have seen benefits from gametophytic selection occurring during pollen-tube growth in the style. It has been proposed that gametophytic selection contributed to the radiation of flowering plants with closed carpels and more efficient pollen transfer by insects enhancing selective pressure on microgametophytes.

Alternative hypotheses to pollen competition

The biological importance of gametophytic selection continues to be a subject of discussion. Suggestions have been made that the significance of the heritable ability of the genes passed on from haploid gametes may not significant and that differences in the number of pollen grains on the stigma or the distance pollen tubes travel through the style may have promoted differences in seed provisioning that resulted in differences in seedling growth instead of heritable genetic differences resulting from pollen competition.

References

{{Reflist Botany