Genetic Diversity in Remnant Swedish Hop (Humulus lupulus L.) Yards from the 15th to 18th Century

Abstract

Hop (Humulus lupulus L.) is a perennial plant cultivated for its use in beer production. The plant is dioecious, and the female plants produce cones containing substances that enhance the taste and durability of beer. Beer was long an essential part of food supply in Northern Europe, and hop has thus been a very important crop during the last 1,000 years. In Sweden, hop cultivation was, by law, mandatory for farmers from 1414 till 1860. Today, Swedish hop cultivation is negligible, but historical remnant hop plants can still be found as feral populations. Using historical maps and documents, we have located ten historical hop yards from the 15th to 18th century where hop plants still persist as now feral populations. Some fifteen plants of each population were sampled and genotyped with ten SSR markers and one marker diagnostic for sex type. In addition, 25 genebank preserved clones of older landraces and cultivars from Europe were genotyped. Genotyping results show abundant clonality and low rates of sexual reproduction within the feral populations. Two of the populations had markedly higher genetic diversity and a higher number of haplotypes, and in these populations a mix of female and male plants was also found. The populations were all clearly differentiated, with no haplotypes shared between populations and little evidence of exchange of genetic material. These results indicate that natural spread and genetic recombination is uncommon or slow in Sweden, and that the feral plants could be remnants of the original historical cultivations. In the assembly of European genebank clones, several clones showed identical genotypes and overall limited genetic diversity. The Swedish populations were in most cases genetically clearly different from the genebank clones. This contrasts with historical records of massive introductions of hop clones from continental Europe during the 19th century and shows that these imports did not replace the original hops being cultivated. A possible better adaption of the Swedish hops and primitive historical breeding are discussed.