Asexual Reproduction

cerned more with the ecological conse- quence of this type of reproduction rather than the genetic or cellular aspects.

Asexual reproduction is of interest to

Concepts

• Asexual reproduction (apomixis) can occur through the production of seeds without fertilization (agamospermy) or clonal reproduction (vegetative growth).

• Agamosperm reproduction has some of the benefits of seed production; however, the lack of genetic recombination means that novel genotypes are not formed and that deleterious mutations may accumulate.

• Agamospermic species vary in their ability to colonize.

• Clonal reproduction allows the individual to bypass the seedling stage of growth lower- ing the mortality of new individuals, but, again, there is no new genetic recombination.

• Reproducing clonally increases a species’ colonizing ability and persistence.

• One species may reproduce via a combination of sexual reproduction, agamospermy and clonal growth, but there is a trade-off of resources among these types of reproduction.

11–57.9%

12–52.9%

5–27.3%

4–51.4%

10–36.4%

8–14.3%

7–66.7%

2–41.2%

3–53.8%

6–39.3%

9–47.8%

1–46.9%

Fig. 5.1.Proportion of the most aggressive non-native species in natural habitats that are capable of clonal growth. Regions are 1) North America n(number of species) =36; 2) Central America; 3) South America n=134; 4) Australasia n=81; 5) Malagassia n=23; 6) Africa n=59; 7) Europe n=24; 8) North Asia n=7; 9) South Asia n=23; 10) Malesia n=12; 11) Pacific n=59; 12) Oceanic Islands n=17 (Pysˇek 1997; with permission of Backhuys Publishers).

Chapter 5

weed ecology, because it allows one individual to invade a new habitat and become established as a population without requir- ing a mate. Many weed species are capable of uniparental reproduction either through self-pollination (see Chapter 4 for exam- ples), agamospermy (e.g. dandelion, Taraxacum officinale) or clonal propaga- tion (e.g. quackgrass, Elytrigia repens) (Barrett, 1992). The importance of asexual reproduction varies with climate and habitat type. Harsh environmental conditions and lack of mates favours individuals that can reproduce asexually. Therefore, the distribution of species that have asexual capabil- ities increases towards the North and South Poles (Pysˇek, 1997) (Fig. 5.1).

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 vari- ations (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-

Sporophyte 2n

Gametophyte

n 2n 2n 2n

Sporophyte 2n

Sporophyte 2n Egg cell

Egg cell 2n

Egg cell 2n Reductional

meiosis

Restitutional meiosis or

meitosis

Reductional meiosis

Fertilization Fertilization

Sexual Apomixis

reproduction Adventitious

Diplospory Apospory embryony

Gametophyte Gametophyte Gametophyte

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 notproceed as usual. As the embryo sac containing the ovum is produced, the numbers of chro- mosome 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 agamospermic 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 devel- opment, 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 agamospermous 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 Hybridization is thought to bring about the conditions necessary for agamospermy. Hybrids may be more vigorous, long-lived and partially sterile.

Polyploidy (multiple sets of Polyploidy ‘may buffer against the effects of deleterious mutations’.

base chromosomes) 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.

Phenotypic plasticity 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.

Polycarpic perennials, Very few annuals, biennials and monocarpic species are agamospermic.

often rosette forming

Pollen limitation 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 agamospermic 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 his- tory strategies in spite of their similar mor- phologies. 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 In the absence of pollination, seed production is assured (although some agamosperms still require the ‘cue’ from pollination to create asexual seeds)

Advantages of seed Obtain dispersal and dormancy but maintain advantages of vegetative reproduction

Avoid ‘cost of meiosis’ No ‘unfit’ zygotes created through recombination that may disrupt co- adapted genotypes. Offspring have same fitness as maternal parent Avoid ‘cost of males’ Energy does not go towards the creation of pollen (although many

agamosperms do produce pollen)

Benefit from ‘extremely Many agamosperms are highly heterozygous and thus have high fitness.

fit genotype’ Less fit genotypes will decrease through natural selection Costs

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

population

No recombination Agamospecies lack genetic recombination which can create novel advantageous genotypes that may be more fit, especially in cases of habitat or climatic changes

Narrow niche 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

Lack ‘fine-tuning’ 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.

Clonal Reproduction

Clonal reproduction (also known as vegetative reproduction, clonal growth and vegetative multiplication) results in the forma- tion of new individuals that are genetically identical to the parent plant and capable of physiologically independent growth (ramets). This differs from the production of

branches or leaves which do not usually persist independently. A genet is the entire genetic individual and is composed of ramets (Fig. 5.3).

Clonality is a highly successful plant strategy and has evolved independently many times in individuals of species that are not otherwise similar or closely related, i.e.

it is a ‘polyphyletic’ trait. Clonal plants have a global distribution. About 28% of dicotyle- dons have some sort of clonal reproduction (Leakey, 1981), and about 40% of these are predominantly clonal. In North America, Ramet

Genet

Ramet Genet

Fig. 5.3.Illustration of ramets of a genet in both: (a) phalanx and (b) guerrilla growth forms.

(a)

(b)

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