Interspecific hybridization among Hieracium species in New Zealand: evidence from flow cytometry

Abstract

Hieracium pilosella (Asteraceae) was accidentally introduced to New Zealand about 100 years ago. Since then it has become an aggressive weed, and an unexpected degree of genetic and genome size variation has been detected; features that might result from interspecies hybridization. We investigated the possibility that H. pilosella has hybridized with related taxa. Of the four other subgenus Pilosella species introduced to New Zealand, H. praealtum is the most abundant and, on morphological and distributional evidence, most likely to be the other parent. Flow cytometry was used to estimate relative genome size for 156 Hieracium plants collected from the wild. Plants assigned to either parental or hybrid morphotypes were found to comprise tetraploid and pentaploid individuals using genome size measurements, and this was confirmed with direct mitotic chromosome counts for a subset of plants. The haploid DNA content of H. praealtum was approximately 22% larger than that of H. pilosella.Putative hybrids that were tetraploid had mean genome sizes equivalent to two H. pilosella and two H. praealtum haploid chromosome sets, implying they were hybrids arising from the fertilization of two reduced gametes. Similar results were obtained from tetraploid hybrids produced by controlled pollination. However, the majority of field hybrids were pentaploid with a genome size equivalent to four H. pilosella and one H. praealtum haploid chromosome sets. We infer that these are not first-generation hybrids but represent successful backcrossing with H. pilosella and/or hybrid-hybrid crossing, and that sexual tetraploid hybrids have been the parents. We note that populations putatively of H. pilosella often comprise apomictic pentaploid hybrids. Significantly, our data indicate the emergence of sexual hybrids that provide further opportunity for gene flow among taxa in this complex.

Enriching Ploidy Level Diversity: the Role of Apomictic and Sexual Biotypes of Hieracium subgen. Pilosella (Asteraceae) that Coexist in Polyploid Populations

“…For example, in a secondary distribution area, the formation of new Hieracium subgen. Pilosella hybrids has been recorded (Lepage 1967;Houliston and Chapman 2004;Morgan-Richards et al 2004;Trewick et al 2004). The presence of hybridization, even in apomicts, most likely facilitated the success of a number of highly invasive biotypes (Chapman et al 2003;Wilson et al 2006).…”

Ploidy levels and reproductive behaviour in invasive Hieracium pilosella in Patagonia

“…In addition, among the herbarium specimens we received from Tierra del Fuego, H. floribundum (a hybridogenous species originated from H. caespitosum and H. lactucella) was also present. All of these species are known to hybridise with H. pilosella in Europe (Sell and West 1976), and at least some of them do so in New Zealand (Morgan Richards et al 2004).…”

“…Pilosella are known as easily hybridising, forming both stabilised hybrids (hybridogenous species) and hybrid swarms, even between different ploidy levels (e.g., Fehrer et al 2007;Sell and West 1976). Efficient hybridisation results in the formation of new forms (either sexual or facultatively apomictic) and increases the evolutionary potential of these species (e.g., Houliston and Chapman 2001;Morgan-Richards et al 2004). …”

Interspecific hybridisation of cytoplasmic male-sterile rapeseed with Ogura cytoplasm and Brassica rapa var. pekinensis as a method to obtain male-sterile Chinese cabbage inbred lines

“…Previously, FCM was used to detect interspecific hybrids obtained from reciprocal crosses between B. napus and B. rapa, as well as B. napus and B. juncea (Sabharwal and Doležel 1993). FCM was also useful for the very early detection of interspecific hybrids within the genera of Lilium (van Tuyl and Boon 1996), Centaurium (Banjanac et al 2014) and Hieracium (Morgan- Richards et al 2004). In our study, the FCM of young, fully expanded leaves revealed that all of the Brassica genotypes were characterised by high degrees of endoreplication.…”