Sexual Reproduction in Asexually Reproducing Species

Clonal reproduction rarely occurs to the total exclusion of sexual reproduction, although there are examples of this, such as Japanese knotweed. Clonal growth may occur at the expense of seed production (creeping buttercup, Ranunculus repens, and Canada goldenrod). A trade-off occurs between these two methods of reproduction because only a finite amount of resources is available to allocate to reproduction (Abrahamson, 1980). The allocation of resources to sexual vs. asexual reproduction will change over the life of the genet. Sexually and asexually produced offspring will have different genetic and ecological characteristics (Table 5.6). For example, offspring produced through sexual reproduction will differ genetically from their parents and, have the ability to disperse, but suffer a high mortality rate in the seedling stage. Offspring produced asexually will develop immediately and have a low mortality rate, but have less dispersal potential.

In some cases there can be individual plants reproducing sexually and asexually alongside conspecific (same species) individuals that are reproducing only one way. For example, individuals of the introduced wild garlic (Allium vineale) produce a stalk which has either only sexually reproducing flowers, only asexual bulbits (bulbs produced on shoots where the flowers normally are located) or both, in addition to producing two types of bulbs at their base (Ronsheim and Bever, 2000). The relative allocation of resources to bulbs, bulbits and flowers is under strong genetic control, as genotypes do not vary allocation patterns in response to nutrient addition.

Seedling recruitment may occur only at some times during the life of a clonal species. For example, seedling recruitment of Canada goldenrod occurred only in the first 3-6 years after colonization, and successful genets were established mainly in the first year (Hartnett and Bazzaz, 1983). This pattern of recruitment is called 'initial seedling recruitment' (ISR). It results in a population with an even age structure because new individuals are recruited at approximately the same time. ISR genets may be long-lived because once established they can be virtually immortal unless a disturbance kills the entire genet.

White clover is an example of the opposite type of recruitment pattern where there is continual recruitment of new genets into the population via seed production (Barrett and Silander, 1992). This type of recruitment is called repeated seedling recruitment (RSR). Such populations have an uneven age structure. Following a disturbance, some genets die making room for new recruitment of genets. RSR genets have

Table 5.7. Expected life-history trends for clonal plants, in relation to their seedling recruitment patterns: initial seedling recruitment (ISR) and repeated seedling recruitment (RSR) (from Eriksson, 1989).

Key features in recruitment phase Dispersal

Genetic diversity in local population Low

Prospects for evolution of locally adapted population Low

Genetic age-structure in local population Even-aged

Genetic life span Long

Spatial context for including genetic population dynamics Large scale

Competitive ability

High

High

Variable

Variable

Small scale shorter life spans because they are continually being replaced (Eriksson, 1993). Of course, many species are likely to be located along a continuum between ISR and RSR. Table 5.7 summarizes the life history traits associated with ISR and RSR patterns.

Case history: Plantain pussytoes - a species with agamospermy, clonal reproduction and sexual reproduction

Plantain pussytoes (Antennaria parlinii) is a herbaceous perennial in eastern North America that reproduces via both sexual and agamospermic seeds and clonally through stolons. Asexual populations tend to be more prevalent in disturbed early suc-

cessional sites (fields and pastures), whereas sexually reproducing populations tend to be in less disturbed sites (open woods and old fields).

Michaels and Bazzaz (1986) compared demographic characteristics and resource allocation of sexual and asexual populations of plantain pussytoes (Table 5.8). Asexual individuals had higher fecundity rates because they produced more, but smaller seeds; however, seedling survivorship was lower. Clonal growth was high in asexual populations with more ramets being produced; however, stolon length and survivorship were decreased. Sexual populations produced long-lived wandering stolons that allowed the genet to persist in spatially and temporally unpredictable envi

Table 5.8. Comparison of seed production, seedling establishment and clonal growth in populations of sexual (female plants only) and asexual (agamospermic) pussytoes (Antennaria parlinii) (from data in Michaels and Bazzaz, 1986, 1989).

Sexual

Agamospermic

Stage

Factor

populations

populations

Seed production

Seed number/inflorescence

252 seeds

389 seeds

Seed massa

approx. 77 |jg

approx. 68 |g

Inflorescences/plant

Fewer

More

Seedling survivorship

Midsummer

47%

22%

End of growing season

7%

4%

Ramet demography

Ramet production (no. ramets/ genet)

2.3

2.8

Ramet survivorship

85%

68%

Stolon length (cm/genet)

8 cm

5 cm

Biomass

Total biomass

More

Less

Allocation to reproduction

Less

More

Response to increase in

Little change in

More towards

resources

biomass allocation

reproduction

aEstimated from graphed data.

aEstimated from graphed data.

ronments. Sexual populations had higher competitive ability, whereas asexual populations had higher fecundity.

Asexual populations of plantain pussy-toes were able to grow in a wider range of nutrient and light levels than sexual populations (Michaels and Bazzaz, 1989). This often is beneficial to colonizing species, especially in environments with unpredictable selection pressures. Furthermore, asexual populations responded to increased resources by allocating proportionally more to increased fecundity and reproductive biomass. Overall, sexual populations of pussy-toes allocated more resources to clonal growth and persistence, and were more competitive than asexual populations. Asexual populations had higher fecundity and were able to respond to changes in environmental resources.

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