Genetic control of the operculum and capsule morphology of Eucalyptus globulus

Abstract

Background and aims: The petaline operculum that covers the inner whorls until anthesis and the woody capsule that develops after fertilization are reproductive structures of eucalypts that protect the flower and seeds. Although they are distinct organs, they both develop from flower buds and this common ontogeny suggests shared genetic control. In Eucalyptus globulus their morphology is variable and we aimed to identify the quantitative trait loci (QTL) underlying this variation and determine whether there is common genetic control of these ecologically and taxonomically important reproductive structures.

Methods: Samples of opercula and capsules were collected from 206 trees that belong to a large outcrossed F2E. globulus mapping population. The morphological variation in these structures was characterized by measuring six operculum and five capsule traits. QTL analysis was performed using these data and a linkage map consisting of 480 markers.

Key results: A total of 27 QTL were detected for operculum traits and 28 for capsule traits, with the logarithm of odds ranging from 2.8 to 11.8. There were many co-located QTL associated with operculum or capsule traits, generally reflecting allometric relationships. A key finding was five genomic regions where co-located QTL affected both operculum and capsule morphology, and the overall trend for these QTL was to affect elongation of both organs. Some of these QTL appear to have a significant effect on the phenotype, with the strongest QTL explaining 26.4 % of the variation in operculum shape and 16.4 % in capsule shape. Flower bud measurements suggest the expression of these QTL starts during bud development. Several candidate genes were found associated with the QTL and their putative function is discussed.

Conclusions: Variation in both operculum and capsule traits in E. globulus is under strong genetic control. Our results suggest that these reproductive structures share a common genetic pathway during flower bud development.

Keywords: Eucalyptus globulus; Beak; QTL analysis; calyptra; candidate genes; capsule; fruit; genetic control; ontogenetic development; opercula; pleiotropy.

Fig. 1.

Traits measured on reproductive structures of Eucalyptus globulus. (A) Measurements on operculum: diameter (OD), height (OH), base height (OB) and beak height (OBK). (B) Measurements on capsule: diameter (CD), height (CH) and wall thickness (CWT) measured on a cut capsule.

Fig. 2.

Longitudinal sections of phenotypically distinctive flower buds of Eucalyptus globulus. On the left, bud with flat operculum beak. On the right, bud with tall operculum beak. Note the coordinated texture, waxiness and colour of the operculum and the receptacle. Scale bar = 5 mm. Arrows show the suture point where the operculum (above the arrows) joins the receptacle.

Fig. 3.

Quantitative trait loci (QTL) associated with operculum and capsule traits in Eucalyptus globulus. Marker positions on the linkage map are shown as lines on the 11 linkage groups. Linkage group number and orientation is as per the E. grandis reference genome (Myburg et al., 2014).

Fig. 4.

Genotype average (h and kk) from the closest marker to the co-located QTL on linkage group 7 (A) and 11 (B) for capsule shape (CS) and operculum shape (both OS and OSC).