Introduction

Plants do not need to rely on sexual reproduction to pass on their genes to the next generation. Asexual reproduction is the creation of new individuals without involving genetic recombination. As a result, asexual offspring are genetically identical to a single parent plant. The ability to reproduce without sex can be a great advantage to organisms (such as plants) that cannot move to find a mate or avoid an inhospitable environment. However, as we will discuss, reproduction without sex has as many costs and benefits as sexual reproduction.

© 2003 CAB International. Weed Ecology in Natural (B.D. Booth, S.D. Murphy and C.J. Swanton)

The two main types of asexual reproduction are clonal growth and agamo-spermy. Clonal growth (or vegetative reproduction) is the creation of new, potentially independent plants through vegetative growth. Agamospermy is the creation of seed without fertilization. The term 'apomixis' is occasionally used as a synonym for agamospermy (Mogie, 1992); however, we prefer apomixis as a synonym for asexual reproduction because we are concerned more with the ecological consequence of this type of reproduction rather than the genetic or cellular aspects.

Asexual reproduction is of interest to nd Agricultural Systems 63

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 (Pysek 1997; with permission of Backhuys Publishers).

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 (Pysek 1997; with permission of Backhuys Publishers).

weed ecology, because it allows one individual to invade a new habitat and become established as a population without requiring a mate. Many weed species are capable of uniparental reproduction either through self-pollination (see Chapter 4 for examples), agamospermy (e.g. dandelion, Taraxacum officinale) or clonal propagation (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 capabilities increases towards the North and South Poles (Pysek, 1997) (Fig. 5.1).

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