Premise: Plant domestication can be detected when transport, use and manipulation of propagules impact reproductive functionality, especially in species with self-incompatible breeding systems.

Methods: Evidence for human-caused founder effect in the Four Corners potato (Solanum jamesii Torr.) was examined by conducting 526 controlled matings between archaeological and non-archaeological populations from field-collected tubers grown in a greenhouse.  Specimens from 24 major herbaria, along with collection records from >160 populations were examined to determine which produced fruits.

Results: Archaeological populations did not produce any fruits when self-crossed or out-crossed between individuals from the same source.  A weak ability to self- or out-cross within populations was observed in non-archaeological populations.  Out-crossing between archaeological and non-archaeological populations, however, produced fully formed, seed-containing fruits, especially with a non-archaeological pollen source.  Fruit formation was observed in 51 of 162 occurrences, with minimal evidence of constraint by monsoonal drought, lack of pollinators, or spatial separation of suitable partners.  Some archaeological populations (especially those along ancient trade routes) had records of fruit production (Chaco Canyon) while others (those in northern Arizona, western Colorado, and southern Utah) did not. 

Conclusions: The present study suggests that archaeological populations could have different origins at different times – some descending directly from large genepools to the south and others derived from gardens already established around occupations.  The latter experienced a chain of founder events, which presumably would further reduce genetic diversity and mating capability.  Consequently, some archaeological populations lack the genetic ability to sexually reproduce, likely as the result of human-caused founder effect.

Breeding Experiment:

Propagation of Source Populations – Tubers from five archaeological (A) and five non-archaeological (N) source populations were obtained directly from 2013-2019 field collections or from accessions stored at the U.S. Potato Genebank, Sturgeon Bay, Wisconsin.  Populations varied greatly in size based on the number of above-ground shoots observed during late summer and early fall.  In all cases, attempts were made to maximize distance between collections within a population to avoid sampling from the same clones.  Tubers were stored at 5°C until grown under optimal greenhouse conditions in April 2020.  Propagation began with 82 tubers from 10 populations (5 A and 5 N) that averaged 71% sprouted (72% and 69%, respectively). Each sprouted tuber was planted in potting soil in a one gallon container and watered with nutrient solution every three days, thus removing any resource constraints to growth, flowering, and fruiting. Flowering began in early July and crosses began on 13 July 2020.    

Crosses – Given that we could not be certain that tubers from a single source population came from different clones, only source populations were considered independent replicates for each type of cross.  Each source was designated a source symbol (e.g. Anc = North Creek Shelter source). Multiple plants (e.g. Anc1, Anc2, etc.) and pollen transfers between flowers within each source population were treated as repeated attempts at each type of cross (self, outcross within a population (W), outcross between populations (B)). Repeated attempts ensured that a particular cross would not fail because of inadequate pollen load on the stigma, indelicate handling of flowers or other uncontrolled factors that inhibited seed and fruit formation. 
We began the breeding experiments with the following types of crosses:
1)    Self – pollen transfers among flowers on the same individual plant of a source population (e.g. Anc1 X Anc1, Nsr1 X Nsr1)
2)    Outcross W – pollen transfers between flowers on different individual plants from within the same source population (e.g. Anc1 X Anc2)
3)    Outcross B – pollen transfers between flowers on individuals from different source populations (e.g. Anc1 X Nsr1).   
Newly opened or unopened flowers receiving pollen were emasculated by removing the stamens and petals, exposing the pistils. Pollen was released from stamens using a mid-C tuning fork onto a piece of clean paper. The paper was then spread evenly over the receiving stigma. A labeled, fine-mesh bag was placed over the pollinated stigma and all tools were then cleaned and sanitized. These were performed between 13 July and 5 August 2020, interspersing cross types among the flowers available at any point in time.  Consequently, the number of replicates and repeats (plant or flowers) could vary as the floral season progressed and the number of available flowers fluctuated.   
Fruits were left to mature and collected when the peduncle senesced and could be easily removed or fell off on its own. All fruits were dried in paper bags at room temperature.

Geography of Fruit Production:

Population database – Field surveys and collections of S. jamesii began in 1992 (U.S. Potato Genebank) and have extended through 2020 (U.S. Potato Genebank and University of Utah).  The current U.S. Potato Genebank database for S. jamesii records the species at 162 geo-referenced locations and includes observations of habitat, population size and condition, tuber collections, and the presence of flowers and/or fruit.  Repeated visits to these locations have resulted in 243 population-specific observations, especially at archaeological sites throughout the range. Sites were subsequently sorted into archaeological vs. non-archaeological and fruit observed or not observed during the surveys.

Herbaria Survey – Seven major herbaria within and beyond the Four Corners region were visited between 2014 and 2020 to examine every available specimen of S. jamesii.  These included: Garrett (UT-Botany); Intermountain (USU-UTC); Deaver (ASC); University of New Mexico (UNM-Vascular Plants); Brigham Young University, S.L. Welsh (BRY-V); Royal Botanic Gardens Kew (KEW); British Museum of Natural History (NHM).  This resulted in 250 specimens examined, with one fruit occurrence added to the population database.
We also examined records and photographs (if available) of specimens of S. jamesii for occurrences of fruit available on the Consortium of Intermountain Herbaria (https://intermountainbiota.org/portal/collections/index.php), which included 17 institutions: ARIZ; ASU-CCH; ASU-Seeds; BLM-BLMGJFO; BLM-NM; COCO; COLO-V; CS; ENMU; FLD; GCNP; Harvard; KAIB; MABA-Plants; MESA; MNA; NAVA; NHI; NMC; NMNHI; NPS-MEVE; NPS-PISP; SJNM; SNM; SUU; SWRS; USFS-COC; USFS-GILA; USFS-SWRH; USFS-TEUI; UVSC; WSCO; YU; ZNP.  This search resulted in an additional 206 specimens, adding three fruit occurrences to the database.  
On average, 86% of all collection records were made during the phenoperiod for fruiting (August through October) and geo-referenced occurrences of flowering and fruiting were mapped in ArcGIS (ESRI, Redlands, California) using U.S. Geological Survey topographic basemap.

See ReadMe file.