A new long-tubed subspecies of Pelargonium dipetalum ( section Hoarea ) ( Geraniaceae ) from the Albertinia-Swellendam area in Western Cape Province , South Africa

http://www.abcjournal.org doi:10.4102/abc.v44i1.163 Background: Field studies confirmed that unusually long-tubed populations of Pelargonium dipetalum from between Swellendam and Albertinia, Western Cape Province, South Africa, are a distinct ecotype adapted to pollination by the long-proboscid fly, Prosoeca longipennis. The geographical and morphological isolation of these populations suggests that they are reproductively isolated from short-tubed populations, which are pollinated by bees.


Introduction
The genus Pelargonium L'Hér.ex Aiton (Geraniaceae) includes approximately 280 species distributed amongst 16 sections (Bakker et al. 2004).Section Hoarea (Sweet) DC., with ± 85 species occurring mainly in the winter rainfall region of South Africa (Marais 2014), is one of the largest sections in the genus.It comprises deciduous geophytes with carrot-shaped or turnip-shaped tubers covered with papery bark and a highly condensed stem with the leaves in a radical tuft and often dry at flowering (Marais 1994(Marais , 2014)).
The section includes seven species with only the posterior two petals present, as opposed to the full complement of five petals commonly found in the genus (Manning & Goldblatt 2012;Marais 1994).Pelargonium dipetalum L'Hér. is the only member of this alliance occurring along the southern coast in Western Cape Province and extends from Betty's Bay to Knysna.The hypanthium in P. dipetalum typically varies between 7 mm and 18 mm in length, but the species also includes a few populations east of Swellendam with exceptionally long hypanthia, 35 mm -50 mm long.These populations were not recognised as a separate taxonomic entity by Marais (1994), who made no correlation between their distribution and their distinctly longer hypanthium.This variant was, however, identified by Manning and Goldblatt (2005) as representing a distinct morph adapted to pollination by the long-proboscid fly Prosoeca longipennis Loew (Nemestrinidae).
A recent study of the P. longipennis pollination syndrome by Newman, Manning and Anderson (2014) defined the guild (Fenster et al. 2004) as including mainly autumn-flowering species with unscented, white to pink flowers with long floral tubes.A population of the long-tubed morph of P. dipetalum included in the study confirmed that it is adapted to pollination by P. longipennis, whereas the short-tubed morph of the species is visited by bees.
We document the distribution and variation in hypanthium length and petal colour in P. dipetalum and confirm that the long-tubed populations represent a variant that is morphologically and geographically discontinuous from the typical populations.We recognise these populations as the new subspecies P. dipetalum subsp.stenosiphon.

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Research method and design
Fieldwork was conducted in the Langeberg area (Western Cape Province, South Africa) and herbarium specimens from the Bolus Herbarium (BOL), Compton Herbarium (NBG) (including the South African Museum [SAM]) and the Pretoria National Herbarium (PRE) were studied (abbreviations according to Holmgren, Holmgren & Barnett 1990).
Hypanthium length and petal colour were recorded for all specimens (Appendix 1).The shortest and longest values for hypanthium length were recorded for each herbarium collection, as well as the mean hypanthium length for all short-tubed and long-tubed collections, respectively.
The distribution map was compiled using the quarter degree reference system (Leistner & Morris 1976).

Results
Populations of P. dipetalum are separable into two groups based on hypanthium length and petal colour.The common form corresponding with the type of the species has shorttubed flowers, with the hypanthium 3 mm -23 mm long (9.7 mm ± 3.1 mm s.d.; n = 144) (Appendix 1; Figure 1), mostly pink petals (rarely white in populations near Knysna and Bredasdorp) (Figure 2) and a widespread distribution along the coast from Betty's Bay to Knysna (Figure 3).Three populations from slightly further inland between Suurbraak and Albertinia have consistently white petals and a significantly longer hypanthium 34 mm -54 mm long (41.1 mm ± 9.0 mm s.d.; n = 8) (Figures 1-3; Appendix 1).There is no overlap in hypanthium length between the two variants.The two variants are geographically separated and are morphologically distinct in hypanthium length and partially in petal colour, but we were unable to find any other differences between them.We accordingly treated the longtubed populations as comprising a separate subspecies representing a distinct ecotype adapted to pollination by the long-proboscid fly P. longipennis.

Distribution
Subsp.stenosiphon is localised along the southern foothills of the Langeberg west of Swellendam, between Suurbraak and Albertinia (Figure 3b).

Conservation
Unlike subsp.dipetalum, which is relatively widespread and partially protected in nature reserves, subsp.stenosiphon has a much more limited range.The taxon is recorded from an area 90 km × 20 km in extent, much of which is under pressure from agricultural activity.No known populations are formally protected and the conservation status of the subspecies needs to be assessed.

Discussion
The two subspecies of P. dipetalum are distinguished by hypanthium length and mostly also by petal colour (Figure 1 and Figure 2).The flowers in subsp.dipetalum are usually pale to dark pink, but two populations, one near Bredasdorp and the other at Belvedere near Knysna, have white flowers, and the hypanthium ranges from 3 mm to 23 mm long.The petals in subsp.dipeatlum are always marked with dark nectar guides.In contrast, the petals in subsp.stenosiphon are consistently white with only faint nectar guides and the hypanthium is 35 mm -54 mm long.The floral differences between the two subspecies of P. dipetalum are associated with a shift in pollination system from bee-pollination in the typical ecotype to pollination by the long-proboscid fly P. longipennis in subsp.stenosiphon.
In an analogous situation, Becker and Albers (2010)

FIGURE 1 :
FIGURE 1: Hypanthium length in the two subspecies of Pelargonium dipetalum, showing the shortest and longest values plus the range in average shortest to average longest values per herbarium collection.

FIGURE 2 :
FIGURE 2: Photographs of Pelargonium dipetalum showing flowers of the two subspecies, (a) Pelargonium dipetalum subsp.dipetalum from Kogelberg with pink flowers and short hypanthium and (b) Pelargonium dipetalum subsp.stenosiphon from Albertinia with white flowers and long hypanthium.
recently described P. alternans subsp.longicalcarM.Becker & F.Albers  (2010)for populations from the Outeniqua Mountains in the Little Karoo differing from the other two subspecies in having a longer hypanthium [(12-)18 mm -36 mm long in P. alternans subsp.longicalcarversus 3 mm -9 mm in subsp.alternansandsubsp.parviinflorescensM.Becker &  F.Albers (2010)].The long-tubed subspecies in this species is also thought to be pollinated by a long-proboscid fly and has undergone evolutionary changes similar to those documented for P. dipetalum subsp.stenosiphon.Hypanthium length in Pelargonium dipetalum subspp.dipetalum and stenosiphon.Measurements are for the shortest and longest hypanthium represented on each herbarium collection studied.