Species delimitation and hybridization history of a hazel species complex

While species is a useful concept in biology, it can be hard to tell where one species starts, and another ends. This can be even more difficult when species hybridise. Zhiqiang Lu and colleagues have examined the problem by investigating a hazel species complex. In Annals of Botany, their new study concludes there are four clusters that could be called species. But hybridisation has left its mark in their genetic make-up.

Hybridisation occurs in many plant communities. It can act as an evolutionary shortcut by increasing genetic diversity. After mixing the genes of radically different parents, the offspring can produce mixtures between the two extremes and open new ecological niches. These new populations can then form new species or else backcross with a parent. Understanding hybridity can help unravel the evolutionary history of a plant population.

Lu and colleagues examined a Chinese hazel species complex (Corylus chinensis–Corylus fargesii). They sampled 322 trees, north and south of the Yangtze River in Central China.

“Because nuts and kernels are important for taxonomical identification, we firstly used 219 specimens with nuts and kernels collected in the field in this study for morphological statistical analyses,” write Lu and colleagues. “A total of 17 traits were finally used. Statistical analyses were then carried out on the morphological traits, and phenotypic clusters were identified through principal component analysis (PCA) using the software PAST 3.0.”

This data on the physical properties of the plant were combined with genotyping. The botanists found that it wasn’t necessary to sequence every single tree. “Because most individuals from the same population always shared the same sequences without intra-population variation, we did not sequence all 322 trees from the 44 populations. In total, we sequenced 260 trees for three DNA fragments. We obtained 520 cpDNA (matK and rbcL) and 538 ITS sequences (including cloned sequences), and aligned the sequence data by manual correction using MAGA v5.”

As well as the plants, the team examined climate data. Analysing what conditions the plants grew in could help reveal their niches. Their results clashed with what was previously thought about hazel in China.

“Our statistical analyses of morphological traits revealed four clusters that are largely inconsistent with previous classifications,” write Lu and colleagues. “For example, cluster A includes populations ascribed to the two varieties of C. chinensis, while some populations previously identified as the original variety of C. chinensis were also placed in clusters B and C, along with those previously identified as the original variety of C. fargesii or var. latifolia. These new phenotypic clusters were delimitated statistically by bark appearance, four bract characters, leaf width, leaf length/width ratio, the number of lateral veins, length of petiole, infructescence shaft, nut length/width ratio and pubescence at the apex of the nut, most of which have been disregarded in previous species delimitations.”

This analysis was confirmed by the genetic data, which identified three clusters, and a fourth may have been backcrossed with an adjacent cluster.

The scientists also write that there were traceable differences in their physical locations. “It is also interesting that the four phenotypic clusters showed large niche differentiation. Four clusters showed complete or partial geographical isolation, with disjunct or largely allopatric distributions. Geographical isolation should have contributed greatly to the inter-cluster isolation of the current distributions. Although the distributional ranges of both cluster C and cluster D overlapped, their preferred altitudes were different, which may result in reproductive isolation, but not complete isolation. Therefore, niche differentiations were distinct between each pair of the four clusters.”

The results show that the diversity of hazel in China is the result of repeated hybridisation. Hazel in China today is a snapshot of a shifting pattern of colonisation, interbreeding and differentiation. The authors hope to study further the extent to which geographical and genetic barriers created these hazel species.