Flora Unveiled

36 i Flora Unveiled

crop species evolved. As noted earlier, the remains of the earliest agricultural settlements
are found in the Fertile Crescent, the broad, arching zone of grassland and woodland that
begins at the Levant and curves around some 2000 km (1,243 miles) eastward to the Zagros
Mountains. However, agriculture arose independently at least six more times in various
regions around the world, including South China, North China, Sub- Saharan Africa, the
South Central Andes, Central America, and the Eastern United States.

The Founder Crops of the Fertile Crescent

Were All Annuals and Selfers

All of the seven founder crops of the Fertile Crescent region have two important features
that made them easier to domesticate than other wild species: they are all annuals, and they
are all self- pollinators, or selfers. Selfers are plants that release their pollen from the anthers
onto the stigmas of the carpel before the opening of the floral bud (“floret”), a process called
anthesis (Figure 3.3A– C). As a result, pollination in selfers occurs prior to anthesis. In con-
trast, nearly all the wild species of plants are “outcrossers”— that is, they require pollen from
other plants to produce seed.
Outcrossing is an evolutionary adaptation in most wild species that ensures genetic mixing,
thus preventing the accumulation of deleterious mutations from one generation to the next.
The build- up of deleterious mutations can lead to the phenomenon of inbreeding depres-
sion—a common occurrence in cereals. However, the eight founder crops are not obligate
selfers. Occasionally, they are pollinated by pollen from nearby plants, so that some genetic
mixing occurs, although on a limited scale. Wind serves as the agent for outcrossing among
the cereals, whereas insects act as vectors for cross- pollination among the pulses and in flax.
The obvious advantage of annuals over perennials for seed crop domestication is that
annuals produce seed in a single growing season; in contrast, perennials typically pass
through a juvenile phase of two or more years before reproducing sexually. The fact that
annuals die after a single growing season also facilitates selection because the death of the
previous generation allows newly selected traits to spread more rapidly in the population.
The advantage of selfing for plant domestication is that it ensures that mutant plants
selected for their favorable traits can be propagated by seed more or less indefinitely, even
when grown in proximity to “wild type” plants. Their habit of selfing makes them nearly
immune to the pollen from the other plants, so the desired trait is seldom diluted in the next
generation by outcrossing. Although the property of selfing may have been an advantage for
plant domestication, it was a major hindrance to the discovery of sex in plants. First, the
process of pollination was completed before anthesis occurred and so was completely con-
cealed within the emerald confines of the unopened floret. Second, there were fewer oppor-
tunities for viewing hybridization events, which might have led to the discovery of sex.

Selection for the Tough Rachis of Domesticated Cereals

The Fertile Crescent region in the Near East is particularly well stocked with wild cereals
suitable for domestication. Of the world’s fifty- six species of large- seeded grasses, thirty- two
grow wild in the Mediterranean area. The wild ancestors of the three core domesticates—
einkorn wheat, emmer wheat, and barley—have all been identified there.^20