Morphological and genetic variation in the Senecio pinnatifolius complex: are variants worthy of taxonomic recognition?

Abstract

The current taxonomy of the Senecio pinnatifolius complex (formerly Australian S. lautus) is inadequate in describing intra-specific variation. We present several putative taxa as alternatives to current subspecies, based on variants observed during both herbarium surveys and field studies. We sought to establish whether these taxa were objectively justified in terms of morphology and genetic relationships. This was done in three ways. First, a morphometric study of plants grown under standard conditions was undertaken. Second, isozymes were analysed to establish genetic relationships within the complex. Third, achene morphology was examined by scanning electron microscopy. Variants from central Queensland (BRIGALOW V.) and the deserts of central and Western Australia (DESERT V.) were clearly separated from all other variants based on the number of involucral bracts. This differentiation into two major groups may warrant subspecific recognition. Although variation within each of the proposed subspecies was continuous, separation of variants was possible based on statistical survey. This is consistent with their formal recognition as varieties. Further work is required to determine correct nomenclature of proposed subspecies and varieties, and to fully elucidate variation and provenance in inland forms.

The genomic basis of parallel ecological speciation

“…Although individuals growing in the same environment present similar morphologies there is also extensive phenotypic variation across geography (Radford et al 2004), suggesting that the colonization of these environments occurred repeatedly. We used a combination of traditional and next generation DNA markers to evaluate whether colonization of coastal environments occurred through the spread of already distinct ecotypes, or through parallel divergence.…”

“…The S. lautus species complex is a group of groundsels endemic to Australia and New Zealand (including Tasmania, Kermadec and Chatham Islands; Ali 1966Ali , 1968Radford et al 2004;Thompson 2005) that has diversified and colonized multiple habitats. In our sampling efforts (see supplementary methods for detailed sampling considerations) we attempted to maximize both geographical and ecological coverage.…”

“…Leaf samples from eight pairs of parapatric (adjacent) populations adapted to sand dunes and rocky headlands were collected in 2008 and 2009 (Roda et al 2013) using sampling sites described in previous studies (Radford and Cousens 2004; Figure 2A). We also collected samples from S. lautus populations adapted to hinterland environments including alpine meadows (3 populations), an arid shrubland, a Brigalow dominated forest, and a rainforest.…”

“…Populations of these plants that grow in similar coastal environments present several morphological similarities (for instance, rocky headland adapted plants are very branched and prostrate while sand dune adapted plants are less branched and erect; Ali , 1968Radford and Cousens 2004) but there is also a large extent of phenotypic variation across geography (Radford and Cousens 2004). A phylogenomic study of a broad collection of S. lautus natural populations revealed that despite the recent diversification of the group, there is a strong geographic stratification of the samples, where parapatric populations are frequently grouped together and present a more heterogeneous genomic divergence than geographically separate populations.…”

“…approach. Contrasting forms of these plants occupy adjacent environments along the southern coast of Australia (Ali 1966(Ali , 1968Radford et al 2004, Thompson 2005.…”