Agamospermy

Agamospermy is the production of seed without fertilization (i.e. the fusion of gametes - sperm and ovum). There are three main types of agamospermy (diplospory, apospory and adventitious embryony) but there are numerous and often complex variations (Fig. 5.2). Normally, meiosis occurs and the gamete (ovum) contains one copy of all chromosomes, i.e. the ovum is 'haploid'. After being fertilized by sperm, the seed will have the normal number of copies of chromosomes (i.e. it will be 'polyploid'). In apospory, meiosis occurs as usual, but the cells that would normally form the ovum degenerate and a polyploid somatic (non-

Sexual reproduction

Apomixis

Diplospory

Apospory

Adventitious embryony

Egg cell n

Fertilization

Egg cell 2n

Egg cell 2n i

Egg cell 2n

Fertilization

Fig. 5.2. Comparisons of sexual and asexual reproduction. Reduced (n) life cycle stage(s) have single border while, unreduced stages (2n) have double borders (redrawn from van Dijk and van Damme, 1999).

Fig. 5.2. Comparisons of sexual and asexual reproduction. Reduced (n) life cycle stage(s) have single border while, unreduced stages (2n) have double borders (redrawn from van Dijk and van Damme, 1999).

sexual) cell replaces the ovum and will form a seed. In diplospory, meiosis does not proceed as usual. As the embryo sac containing the ovum is produced, the numbers of chromosome copies are not reduced as is normal. The result is an ovum that has at least two (and usually more) complete copies of all chromosomes and this will form the seed. In adventive embryony, meiosis also does not proceed as normal and is altered so much that both embryo sac and ovum are not produced, and somatic cells form the embryo directly. In some cases, an asexual and sexual embryo can occur in the same seed because the normal sexual processes may still occur (van Dijk and van Damme, 2000).

Agamospermy is very common among ferns. It is not present in gymnosperms and it is present in only about 10-15% of angiosperm families (Richards, 1997). In angiosperms, approximately 75% of agamo-spermic taxa are in the daisy (Asteraceae), grass (Poaceae) and rose (Rosaceae) families. A high proportion of the species in the dandelion (Taraxacum), hawkweed (Hiera-cium), and raspberry (Rubus) genera are agamospermic.

Facultative agamospermy is the production of asexual seeds if pollination fails. It is present in some cinquefoils (Potentilla). This trait is particularly useful to weeds because they can produce seeds both with and without pollen, and this can aid the spread of a species when pollinators are absent in the new habitat. For example, the dioecious species screwpine (Pandanus tec-torius) was able to invade islands because it could produce agamospermic seeds and therefore male plants were not necessary for it to colonize (Cox, 1985).

Obligate agamosperms are only able to produce seeds asexually; however, agamospermy rarely occurs to the total exclusion of sexual reproduction. Many raspberries and most dandelions are obligate agamosperms. Species with obligate agamospermy are often triploids or pentaploids and therefore cannot reproduce via pollen.

The occurrence of agamospermy is often associated with the following traits or conditions: polyploidy, phenotypic plasticity, perennial habit, hybridization and pollen limitation (Table 5.1). These traits do not necessarily cause agamospermy to develop: they may be either conducive to its development, or occur as a result of it. In some cases, the association of agamospermy with these traits is not fully understood. For example, some perennial weeds are agamo-spermous whereas others are not, and it is not always clear why a particular species has developed this trait.

Table 5.1. Traits and conditions associated with agamospermy (based on information from Asker and Jerling, 1992 and Richards, 1997).

Trait or character

Description

Hybridization

Polyploidy (multiple sets of base chromosomes)

Phenotypic plasticity

Polycarpic perennials, often rosette forming Pollen limitation

Hybridization is thought to bring about the conditions necessary for agamospermy. Hybrids may be more vigorous, long-lived and partially sterile.

Polyploidy 'may buffer against the effects of deleterious mutations'. Polyploidy is also associated with other changes such as altered secondary metabolism, increased seed size and seedling vigour, and a switch from annual to perennial habit.

As in inbreeding population, agamospermic species tend to have higher phenotypic plasticity. Selection is more likely to encourage phenotypic plasticity in populations with less genetic variability.

Very few annuals, biennials and monocarpic species are agamospermic.

When seed production is limited by the lack of pollen, seeds produced by individuals carrying an agamospermic mutation are more likely to persist in higher numbers.

Costs and benefits

Richards (1997) proposed several costs and benefits of agamospermy. Agamosperm reproduction has some of the benefits of sexual reproduction (e.g. seed production), often without the costs of pollen production (Table 5.2). While agamospermy may avoid the cost of meiosis, the lack of recombination means that deleterious mutations may accumulate and novel genotypes are not formed. Not every cost and benefit will apply to all agamosperm species. For example, some agamosperms require pollen chemicals to help form the endosperm, although the pollen's gametes are not used in the creation of the new individual (e.g. blackberry, Rubus fruticosus).

Ecological aspects

We have said that possessing the ability to reproduce via agamospermy can improve the chances of colonization success and gave screwpine as an example. Not all agamo-spermic species are equally good colonizers. While agamospermy increases the chance of colonization, other traits are required. For example, two closely related species of agamospermous dandelion (Taraxacum), which co-occur in sand dunes of Northumberland, UK, have different life history strategies in spite of their similar morphologies. Rock dandelion (Taraxacum lacistophyllum) is more opportunistic than Taraxacum brachyglossum because it has a faster growth rate, shorter life span, reproduces earlier, has lighter and more dispersible seeds, and can respond faster to the addition of nutrients (Ford, 1985). Thus, rock dandelion is a more successful

Table 5.2. Costs and benefits associated with agamospermy (based on text in Richards, 1997).

Description

Benefits Assured reproduction

Advantages of seed

Avoid 'cost of meiosis'

Avoid 'cost of males'

Benefit from 'extremely fit genotype'

Costs

Accumulate detrimental mutations

No recombination Narrow niche

Lack 'fine-tuning'

In the absence of pollination, seed production is assured (although some agamosperms still require the 'cue' from pollination to create asexual seeds)

Obtain dispersal and dormancy but maintain advantages of vegetative reproduction

No 'unfit' zygotes created through recombination that may disrupt co-

adapted genotypes. Offspring have same fitness as maternal parent Energy does not go towards the creation of pollen (although many agamosperms do produce pollen) Many agamosperms are highly heterozygous and thus have high fitness. Less fit genotypes will decrease through natural selection

Non-lethal detrimental mutations will remain in the population because there is no recombination and selection to remove them from the population

Agamospecies lack genetic recombination which can create novel advantageous genotypes that may be more fit, especially in cases of habitat or climatic changes Outcrossing creates genetic variation among individuals of a population that will lead to increased likelihood of inhabiting more niches. This is lacking in agamosperm populations, although there is some evidence of high levels of somatic mutations in asexual lineages Recombination can create genotypes better adapted to local environments. Agamosperm populations are more likely to be generalists (weedy)

colonizer even though both species are agamospermic.

0 0

Responses

  • Margaret
    Is agamospermic individuals are haploids?
    2 years ago

Post a comment