The domestication syndrome in vegetatively propagated field crops

Abstract

Background: Vegetatively propagated crops are globally significant in terms of current agricultural production, as well as for understanding the long-term history of early agriculture and plant domestication. Today, significant field crops include sugarcane (Saccharum officinarum), potato (Solanum tuberosum), manioc (Manihot esculenta), bananas and plantains (Musa cvs), sweet potato (Ipomoea batatas), yams (Dioscorea spp.) and taro (Colocasia esculenta). In comparison with sexually reproduced crops, especially cereals and legumes, the domestication syndrome in vegetatively propagated field crops is poorly defined.

Aims and scope: Here, a range of phenotypic traits potentially comprising a syndrome associated with early domestication of vegetatively propagated field crops is proposed, including: mode of reproduction, yield of edible portion, ease of harvesting, defensive adaptations, timing of production and plant architecture. The archaeobotanical visibility of these syndrome traits is considered with a view to the reconstruction of the geographical and historical pathways of domestication for vegetatively propagated field crops in the past.

Conclusions: Although convergent phenotypic traits are identified, none of them are ubiquitous and some are divergent. In contrast to cereals and legumes, several traits seem to represent varying degrees of plastic response to growth environment and practices of cultivation, as opposed to solely morphogenetic 'fixation'.

Keywords: Asexual (clonal) reproduction; archaeobotany; developmental plasticity; early agriculture; phenotype; vegetative propagation.

Fig. 1.

Loci of domestication for globally significant food crops (upper; after Fuller et al., 2014: fig. 1) and annual global production (lower; FAO, 2016) for major agricultural crops grown in fields (monoculture) and plots (polyculture). Arboricultural/silvicultural crops, such as trees, palms and pandanus, and fodder crops are excluded. Groups of crops are colour-coded according to: sexually reproduced cereals (blue); sexually reproduced legumes and vegetables (green); and vegetatively propagated bananas, root crops and sugarcane (orange). Notes: 1. In the map (upper), an asterisk connotes that plants probably moved as a weed from region of origin and domesticated in another locale; oats (Avena sativa) and rye (Secale cereale) originated in South-west Asia and were probably domesticated in eastern-central Europe during the late Holocene. 2. In the graph (lower), yield of sugarcane may represent total crop biomass, while other crops are usually given as primary product only.

Fig. 2.

Comparison of the unilinear domestication episode for barley (upper; Hordeum vulgare) with the multistaged and stepwise domestication trajectory for bananas (lower; Musa cvs). Upper: the domestication episode for barley (Hordeum vulgare) potentially extends from approx. 11 500 cal BP to approx. 8500 cal BP and is reconstructed from archaeobotanical evidence at multiple sites for percentages of non-shattering cultivars (red) and increasing grain breadth (blue; Fuller et al., 2014: tables S2 and S3). Lower: rates of change in domesticatory traits are inferred to increase across three thresholds: early planting of diploid cultiwilds; hybridization to generate diploid cultivars; and triploidization with subsequent widespread dispersal (Perrier et al., 2011; De Langhe et al., 2015).

Fig. 3.

Microfossil techniques for the investigation of vegetatively propagated crops. (A) Photomicrograph of starch granule of Disocorea hispida indicating diagnostic elements: h = hilum, l = lamellae (modern reference sample). (B) Photomicrograph of volcaniform phytolith of an AAw banana (Musa sp.; modern reference sample from Ngezi Forest, Pemba). (C) Photomicrograph of a transverse section through a sugarcane (Saccharum officinarum) stem fragment from a 200- to 300-year-old domestic context at Kuk Swamp (Lewis et al., 2016: fig. 3d).

Fig. 4.

Archaeobotanical techniques for the investigation of calcium oxalate in taro (C. esculenta). (A) MicroCT visualization of a parenchyma fragment with low density areas in blue (cell walls) and high density areas in red (druses and raphide bundles comprising calcium oxalate crystals). (B) MicroCT visualization showing the distribution of high density areas in (A). (C and D) Scanning electron microscopy images of taro parenchyma with druses visible as lighter concentrations. (E) Photomicrograph of a cell packed with raphides. (F) Photomicrograph of a raphide showing needle-like morphology and an asymmetric proximal tip (lower right). All images are from modern reference samples.