Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 464

Table 10.3. Breeding system, genetic variation, and life history

Breeding Formation ofnew

system Heterozygosity gene combinations Life history

Outbreeding High Fast Mostly wandering perennials

Inbreeding Low Slow Mostly annuals and stationary perennials

Asexual High Very slow Wandering perennials and apomicts

Source : Modified from Clegg & Brown (1983).

random assortment of chromosomes obtained from different parents and
crossing-over between chromosomes bearing different alleles. In weeds, out-
breeding may allow rapid differentiation of invading populations in response
to newly encountered conditions (Warwick, Thompson & Black, 1987). A
variety of mechanisms have evolved to insure a high rate of outbreeding in
plants.These include self-sterility genes (e.g., Trifolium repens - Atwood, 1940,
1942), differences in the timing of pollen shed and style receptivity (e.g.,
Daucus carota - Bell, 1971), and dioeciousness (e.g., Rumex acetosella - Baker,
1962), though the last mechanism is rare in weeds.
Repeated selfing tends to increase random loss of variability. Selfing
decreases heterozygosity (Solbrig & Solbrig, 1979, pp. 168-9) and this
increases the chance that rare alleles will be lost. Moreover, crossing-over
during meiosis in a selfing event rarely creates new gene combinations,
because the chromosomes involved are usually identical.
An important consequence of selfing is that well-adapted gene combina-
tions are protected against disruption during reproduction (Stebbins, 1957).
This and the ability to propagate at low density have probably selected for
selfing in many weed species that lack vegetative reproduction.
Asexual plants have only limited ability to generate new genotypes.
Somatic mutation may occasionally add variability to the population, and
even basically asexual species like Taraxacum officinale outcross occasionally
(Richards, 1970). Nevertheless, generation of new genotypes in most asexual
species is probably very slow. However, the principal process that creates
sterile plant populations, namely interspecific hybridization, necessarily
results in high levels of heterozygosity.This can confer competitive ability and
broad adaptation via heterosis if the two genomes are compatible with respect
to vegetative characters. Examples of highly successful asexual weeds include
Oxalis pes-caprae , Taraxacum officinale , and Erigeron annuus (Baker, 1965; Solbrig,
1971; Stratton, 1991). Genetic evidence indicates that Cyperus rotundus is also
essentially asexual, with single clones covering large regions (Okoli et al .,
1997).