and a third the size of fruit and seed (Fig. 7.5). Doebley et al. (1990) found five of the QTL that distinguish maize and teosinte in a tight cluster on chro- mosome 8. These genes regulated: (i) the tendency of the ear to shatter; (ii) the percentage of male spikelets in the primary inflorescence; (iii) the aver- age length of internodes on the primary lateral branch; (iv) the percentage of cupules lacking the pedicellate spikelet; and (v) the number of cupules in a single rank. Many of the genetic factors controlling domestication-related traits were also concentrated on a few chromosomal blocks in pearl millet (Poncet et al., 1998), tomato (Grandillo and Tanksley, 1996) and rice (Xiong et al., 1999).
Rye and oats are thought to have begun as weeds of the Near Eastern assemblage, which were domesticated in more northern climates where their potential was more apparent. Hexaploid wheat may also have begun its existence as a weed in cultivated tetraploid fields before its full benefit was noted.
As farming progressed, humans also began to inadvertently select for weedy crop mimics through tilling, weeding and harvesting (Harlan et al.
1973). As we discussed in Chapter 5, many of these weedy races arose after introgression with the crop type, but many non-crop species have also developed weedy races. Two major forms of crop mimics are generally rec- ognized: vegetative, where the weed is similar looking to crop seedlings and their vegetative stage; and seed, where weed seeds have similar density and appearance to those of crops, making it difficult to separate them before planting.
Numerous examples of crop mimicry have been reported. Several species of wild rice and barnyard grass have evolved developmental and Fig. 7.5. Linkage map location of known genes and marker loci controlling the domestication syndrome in common bean. Symbols for the genes: fin, determinacy;
P, anthocyanin pigmentation; Ppd, photoperiod-induced delay in flowering; St, pod string; y, yellow pod colour. Symbols for the marker loci: DF, days to flowering;
DM, days to maturity; DO, seed dormancy; HI, harvest index; L5, length of the fifth internode, NM, number of nodes on the main stem; PL, pod length; NP, number of pods per plant; PD, photoperiod induced delay in flowering; SW, seed weight.
(Used with permission from E.M.K. Koinange, S.P. Singh and P. Gepts, 1996, Crop Science36, 1037–1045.)
growth patterns that make them very difficult to distinguish from cultivated types (Barrett, 1983). In the case of barnyard grass, the crop mimic Echinochloa crus-gallivar. oryzicolais actually more similar to rice in many attributes than to its own progenitors (Barrett, 1983). Weedy races of grain chenopods, teosinte, amaranths, pearl millet and sorghum invade agricul- tural fields and look almost identical to their related crop species until their inflorescences shatter just before harvest (Sauer, 1967; Harlan et al.,1973;
Wilson and Heiser, 1979). Considerable differentiation is often observed in weedy races depending on the types of cultivars grown in a region and the natural diversity present. This is particularly apparent in weedy rice, where there are numerous different indica- and japonica-mimicking races, both where wild species are present and where they are absent (LingHwa and Morishima, 1997; Xiong et al., 1999). One of the most unusual adaptations has been described in maize fields in Mazatlan, Mexico, where fields are cropped one year and then fallowed the next. Here teosinte populations have arisen with an inhibitor that prevents germination for 1 year and there- fore protects the plants from being grazed in the fallow years (Wilkes, 1977).
Numerous examples of seed mimicry have also been described. One of the earliest cases involved races of Camelina sativa, whose seeds were so similar to those of flax that they could not be separated by winnowing, where chaff and lighter seeds are removed by wind from the crop’s heavy seeds (Stebbins, 1950). Seeds of Vicia sativaare normally a different shape and size from lentils, but in Central Europe the seeds of the two species are very similar and as a result V. sativacan be a very serious weed (Rowlands, 1959). In many cases, vegetative and crop mimicry are combined in weeds, making it almost impossible to identify the invaders. The Camelina species have the same growth habit, branching pattern, flowering time and fruit characteristics as flax.
With the advent of herbicides, a third class of mimics has arisen: her- bicide mimicry. Resistance to S-triazine herbicides, atrizine and simizine, has been found in several different weed species, including Brassica campestris and Chenopodium alba (Souza Machado et al., 1977;
Warwick and Black, 1980). The distribution is highly localized in most cases, but in Senecio vulgaris a wide range of susceptibility was found to simazine among the fruit farms in England. A population’s susceptibility was correlated with the number of years of continuous herbicide use, strongly implicating selection (Holliday and Putwain, 1980); however, continuous selection with herbicides has not always been necessary for resistance to emerge. Friesen et al. (2000) found native populations of Avena fatuain Manitoba, Canada, that were resistant to imazamethabenz, even though this herbicide had not previously been applied. Numerous factors influence the rate that herbicide resistance evolves, including rates of genetic mutation, initial frequency of resistance genes, type of inheri- tance, mating system and gene flow (Jasieniuk and Maxwell, 1996;
Mortensen et al., 2000).