Free Axial Lobes: An Important Diagnostic Character in Pteridium (Dennstaedtiaceae)

Abstract

The overall form of the lamina in the bracken fern (Pteridium) is a fractal series in which blade, pinnae, pinnules and pinnulets represent a hierarchy of repeated units of decreasing size. Distinctive lunate or semilunate lobes of laminal tissue between the divisions of the blade in an apical zone of some or all of its axes are an important diagnostic feature. These "free lobes" link Pteridium esculentum (G. Forst.) Cockayne subsp. esculentum from Australasia with P. esculentum subsp. arachnoideum (Kaulf.) J.A. Thomson from Central and South America and separate both from P. aquilinum (L.) Kuhn. The development, pattern of distribution on laminal axes and variation of form of the free lobes in P. esculentum subsp. esculentum are described here. Although basic frond architecture as reflected in pinna spacing on the rachis is largely independent of the size of the lamina, the number and pattern of occurrence of free lobes is significantly correlated with the length of axial intervals and hence with frond size and laminal dissection. Attention is drawn to the likelihood that a genetic polymorphism may underlie development of free lobes in P. esculentum subsp. arachnoideum. The presence or absence of free axial lobes may be a useful marker for detection of introgression following hybridisation of P. esculentum and P. aquilinum.

Striking genetic homogeneity in the widespread South American bracken

“…For example, in P. aquilinum , a species widely distributed in the Northern Hemisphere from North America and across Eurasia and sub‐Saharan Africa, Der et al (2009) and Wolf et al (2019) both found molecular support for most distinct morphological subspecies (Thomson et al, 2005, 2008), which at the same time corresponded with geographic location. A similar pattern could be expected for Southern Hemisphere P. esculentum at the subspecific level (Thomson, 2016), but our findings in the present study suggest that the story may be more complicated.…”

“…Thomson (2016) suggested that the observed morphological variability within Pteridium is likely related to genetic polymorphisms. Wolf et al (2019) found population structure correlating to the subspecies of P. aquilinum , but detected no population structure within South American and Australian populations of P. esculentum .…”

“…In particular, the Northern Hemisphere diploid, P. aquilinum, is known to have many subspecies based on morphological, molecular, and geographic differences. By contrast, in the Southern Hemisphere diploid, P. esculentum, only a handful of subspecies have been described using morphological and habitat differences (Thomson, 2016;Schwartsburd et al, 2014Schwartsburd et al, , 2017Schwartsburd et al, , 2018. A molecular approach to the systematics of P. esculentum is a necessary step toward understanding the disparate biology and ecology of widespread species.…”

Ants associated with fronds of the tropical bracken fern Pteridium esculentum subsp. arachnoideum

“…Pteridium (bracken fern) is a fern genus distributed world-wide. The circumscription of species is widely discussed in the literature, about 20 morphotypes have been recognized, and several infra-specific ranks have been adopted (PPG I 2016, Thomson 2016. While some authors recognize only two species: Pteridium aquilinum (L.) Kuhn and P. esculentum (G.…”

“…Forst.) Cockayne along their infraspecies (Thomson 2016). All the species of the genus have nectaries on their fronds (fern leaves) (Tryon & Tryon 1982), and the oldest record of nectaries in ferns was reported by Francis Darwin in 1877 for Pteridium aquilinum (L.) Kuhn (Lloyd 1901).…”