Corylus avellana - EUFORGEN European forest genetic resources programme

Genetic summary

Common hazel has been shown to have the highest level of genetic diversity in Europe compared with other deciduous species such as hawthorn (Crataegus monogyna), ash (Fraxinus excelsior), sessile oak (Quercus petraea), pedunculate oak (Quercus robur), and alder (Alnus glutinosa) (Brown et al., 2016). Its morphology is also highly polymorphic, with previously described species such as filbert hazel (Corylus maxima), pontic hazel (Corylus pontica) and colchian hazel (Corylus colchica) now being included as subspecies of common hazel (Brown et al., 2016).

In Finnish populations 83% of genetic diversity is within populations, and a lack of genetic structure in these populations suggests high gene flow (Tanhuanpää et al., 2019). Natural populations of common hazel in the United Kingdom and Ireland also have high levels of genetic diversity and low levels of population differentiation, indicating high effective gene flow across the populations and low asexual reproduction (Brown et al., 2016). It would be expected that gene flow is low as hazel nuts are heavy so dispersal is limited; however, the nuts can be dispersed far by birds, with nutcrackers (Nucifraga) having been observed to transport seeds up to 22 km, and pollen dispersal is high (Brown et al., 2016). Common hazel has a weak but significant correlation between genetic and geographic distances and absence of population structure, showing genetic uniformity across the recolonized part of the species’ range (Brown et al., 2016).

Common hazel can reproduce vegetatively, allowing it to create clonal individuals, which typically reduces overall genetic diversity, but vegetative reproduction appears to be rare in wild populations of common hazel (Brown et al., 2016).

The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2024.

Further relevant genetic traits

Glacial biogeography evolution

Pollen records suggest common hazel was one of the first species to recolonize Europe after the last ice age, replacing earlier colonizers such as birch (Betula) and then becoming an understory species, with its most northern populations in Fennoscandia (Brown et al., 2016). The recolonization of northern Europe by common hazel likely came from southwestern refugia, with Italian and Balkan refugial populations remaining in situ (Brown et al., 2016).

Cultivation and human intervention

Common hazel is diploid, monoecious, hermaphroditic, and self-incompatible, and has dichogamy and sporophytic incompatibility to avoid self-fertilization, making it strictly cross-pollinated. This also makes intraspecific hybridization common and means it is typically clonally propagated in cultivation (Brown et al., 2016; Helmstetter et al., 2020).

Its current distribution is heavily influenced by human-mediated dispersal, with Mesolithic tribes likely spreading the species even if they did not cultivate it (Helmstetter et al., 2020). Genetic and archaeological data suggest common hazel was domesticated multiple times (Brown et al., 2016). Common hazel is now the sixth most economically important tree nut crop worldwide. Italy produces roughly 15% of the world’s hazel nuts, and Türkiye producing as much as 80% (Helmstetter et al., 2020). Therefore, there is interest in breeding for commercially valuable cultivars and varieties resistant to disease and fungus such as Eastern filbert blight (Anisogramma anomola) (Rowley et al., 2018). Common hazel is the first genetically sequenced plant in the order Fagales (Rowley et al., 2018).

Domestication and selective breeding often cause bottlenecking and reduce genetic diversity. However, this may not be the case for perennial species such as hazelnut, as similar levels of heterozygosity were found in cultivated and wild individuals in Türkiye (Helmstetter et al., 2020). However, cultivated populations in Türkiye show more structuring and clustering than wild common hazel, with cultivars assigned to groups based on morphology such as nut shape (Helmstetter et al., 2020). There is evidence of past gene flow between wild and domesticated common hazel as well as among different cultivars. This can be problematic as gene flow between wild and cultivated varieties can lead to extinction of the wild species or poorer yields of cultivated varieties (Helmstetter et al., 2020).

The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2024.

Genetic conservation

Threats to genetic diversity and gene flow of common hazel include habitat loss due to urbanization, land-use change, climate change, and diseases and fungus such as Eastern filbert blight (Brown et al., 2016). Additionally, the introduction of non-native species poses a risk of genetic pollution through hybridization. Recently a new powdery mildew disease has emerged affecting trees in the Black Sea region threatening Türkiye’s hazelnut production while also threatening American cultivars (Helmstetter et al., 2020). Therefore, controlled management of domestic common hazel varieties to reduce genetic pollution or the spread of diseases may be prudent. However, there is a lack of research on the conservation of common hazel genetic variation or the management of genetic conservation units.

The bibliographic review was conducted by James Chaplin of the EUFORGEN Secretariat in August 2024.

Corylus avellana
Common hazel

Environmental zones

Status of Corylus avellana conservation in Europe

Genetic summary

Further relevant genetic traits

Glacial biogeography evolution

Cultivation and human intervention

Genetic conservation

Genetic Characterisation of Corylus avellana and its GCUs

Availability of FRM

Contacts of experts

Further reading

References

Corylus avellanaCommon hazel

Environmental zones

Status of Corylus avellana conservation in Europe

Genetic summary

Further relevant genetic traits

Glacial biogeography evolution

Cultivation and human intervention

Genetic conservation

Genetic Characterisation of Corylus avellana and its GCUs

Availability of FRM

Contacts of experts

Further reading

References

Corylus avellana
Common hazel