A natural hybrid of Sindora (Fabaceae, Detarioideae) from Singapore

Abstract

Sindora×changiensis L.M.Choo, Loo, W.F.Ang & K.Er is a new hybrid from the subfamily Detarioideae in Fabaceae. This is the first reported instance of natural hybridisation in Sindora. Based on population genetics analyses using ddRAD and morphological observations, this taxon represents a fertile hybrid between Sindoracoriacea and Sindoraechinocalyx. This new hybrid is so far only known to occur naturally from Changi at the north-eastern coast of Singapore. It has pods that are sparsely spiny. This is intermediate between the smooth, non-spiny pods of S.coriacea, and the densely spiny pods of S.echinocalyx. The calyx is smooth and unarmed, resembling S.coriacea. Last but not least, the ovary is entirely pubescent, different from S.coriacea and S.echinocalyx. The ovary of S.coriacea has a glabrous patch in the middle, while that of S.echinocalyx has minute spines protruding from the dense pubescence. A taxonomic description and an updated key to the Sindora of Singapore and Peninsular Malaysia are also provided.

Keywords: Caesalpinioideae; Changi; Sindoracoriacea; Sindoraechinocalyx; ddRAD; new hybrid.

Figure 1.

A neighbour-net plot with uncorrected p-distances, constructed in SPLITSTREE using the SNP dataset of all 14 individuals of Sindora sequenced.

Figure 2.

STRUCTURE plot generated for the optimal value of K = 2. Samples identified as Sindoracoriacea are wholly yellow, Sindoraechinocalyx samples are wholly blue, while the hybrid individuals of Sindora×changiensis have a mix of both yellow and blue.

Figure 3.

The hybrid tree Sindora×changiensisA overview of the 27 m tall tree B developing (green) and ripe (brown) pods on the tree C inflorescences of the tree. (Photos: A K.B.H. Er, B, C L.M. Choo).

Figure 4.

Comparisons of leaf and flower characters between Sindora×changiensis and its parent species. A, B, C leaves of (A) S.coriacea, (B) S.×changiensis and (C) S.echinocalyx respectively D, E, F flowers of (D) S.coriacea, (E) S.×changiensis and (F) S.echinocalyx respectively, showing the unarmed calyces of S.coriacea, S.×changiensis, and the spiny calyx of S.echinocalyxG, H, I Ovaries of (G) S.coriacea, (H) S.×changiensis and (I) S.echinocalyx respectively, showing the glabrous patch in the centre for S.coriacea; the densely pubescent ovary for S.×changiensis except for the three stripes across the width; and the densely pubescent ovary for S.echinocalyx, with tiny protuberances visible on the surface, which will later on develop into the spines on the fruit pods. Scale bars: 1 cm (A, B, C); 1 mm (D, E, F, G, H, I). (Photos: L.M. Choo).

Figure 5.

Comparisons of pod and seed characters between Sindora×changiensis and its parent species A, B, C pods of (A) S.coriacea, (B) S.×changiensis and (C) S.echinocalyx respectively, showing the unarmed pod of S.coriacea, the sparsely spiny pod of Sindora×changiensis, and the densely spiny pod of S.echinocalyxD, E, F seeds of (D) S.coriacea, (E) S.×changiensis and (F) S.echinocalyx respectively. Scale bars: 1 cm (A, B, C, D, E, F). (Photos: L.M. Choo).

Figure 6.

Aerial photo of the Changi area from various years showing Sindora×changiensis circled in yellow A aerial photo on 17 Feb 1946, which is the earliest archival aerial photo of the Changi area, showing the tree as part of the rainforest remnant B aerial photo on 14 Jul 1950, showing the erection of the Cranwell bungalows to the left and right of the tree C aerial photo on 5 Mar 1963 D present day aerial photo showing the tree. (Images: A–C aerial photographs by the British Royal Air Force between 1940 to 1970s, from a collection held by the National Archives of Singapore. Crown copyright, reproduced in part D imagery 2021 Maxar Technologies, Map data 2021 Google).