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Title: Trichome morphology and development in the genus Antirrhinum
Author: Tan, Ying
ISNI:       0000 0004 7654 678X
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2018
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The distribution of epidermal hairs (trichomes) is an important taxonomic character in the genus Antirrhinum. Most species in subsection Antirrhinum produce trichomes from lower internodes and leaves, then have bald stems and leaf blades after the third node and resume trichomes production again in the inflorescence (the "bald" phenotype). All species in subsection Kickxiella produce trichomes throughout development (the "hairy" phenotype). Populations of some species are polymorphic for trichome distribution-both bald and hairy individuals were observed in A. australe, A. graniticum, A. latifolium and A. meonanthum. Antirrhinum species also varied in trichome morphology. Five types were recognized according to length and the presence or absence of a secretory gland. Some types were present in all species and had similar distributions-for example short glandular trichomes were found on the adaxial midribs of all leaves in all species, and the lower leaves and internodes of all species shared longer glandular and long eglandular trichomes. However, the trichomes on leaf blades and stems at higher vegetative nodes of hairy species and in the inflorescences differed in morphology between species, suggesting that they are regulated differently from trichomes at more basal positions. Other species in the tribe Antirrhineae showed similar variation in trichome morphology and distribution to Antirrhinum, suggesting that the control of trichome development might be conserved within the tribe. To understand the genetic basis for variation in trichome distribution, a near-isogenic line (NIL) was generated by introducing regions of the genome of A. charidemi (hairy, subsection Kickxiella) into the genetic background of A. majus subsp. majus (bald, subsection Antirrhinum). One NIL segregated bald and hairy progeny, with the same trichome distributions as the parent species, in a ratio that suggested a single locus is responsible for the differences and baldness is dominant. The locus was named as Hairy and assumed to act as a suppressor of trichome formation. Progeny of the NIL were used in genome resequencing of bulked phenotype pools (Pool-seq) to map Hairy. No recombination between Hairy and a candidate gene (GRX1) from the Glutaredoxin gene family, was detected in the mapping population. In addition, RNA-seq revealed that GRX1 was expressed in bald parts of bald progeny, but not in the same parts of hairy progeny, and in situ hybridisation showed GRX1 RNA was restricted to epidermal cells, which form trichomes in the absence of Hairy activity. A virus-induced gene silencing (VIGS) method was also developed to test GRX1 function further. Reducing GRX1 activity allowed ectopic trichome formation in the bald NIL. Together, this evidence strongly supported Hairy being GRX1. To investigate evolution of Hairy and its relationship to variation in trichome distribution, the NIL was crossed to other Antirrhinum species. These allelism tests suggested that Hairy underlies variation in trichome distribution throughout the genus, with the exception of A. siculum, which has a bald phenotype but might lack activity of hairy and a gene needed for trichome formation. Hairy sequences were obtained from representative of 24 Antirrhinum species and two related species in the tribe Antirrhineae. The conserved trichome-suppressing function of the sequence from one of these species (Misopates orontium, bald phenotype) was confirmed by VIGS. Gene phylogenies combined with RNA expression analysis suggested that the ancestral Antirrhinum had a bald phenotype, that a single mutation could have given rise to the hairy alleles in the majority of Kickxiella species, that these alleles were also present in polymorphic populations in the other subsections, consistent with transfer from Kickxiella by hybridisation, and that multiple, independent mutations had been involved in parallel evolution of the hairy phenotype in a minority of Kickxiella species. Phylogenetic analysis of GRX proteins suggested that Hairy gained its trichome-repressing function relatively late in the evolutionary history of eudicots, after the Antirrhineae-Phrymoideae split, but before divergence of the lineages leading to Antirrhinum and Misopates. A yeast two-hybrid screen identified members of the TGA and HD-Zip IV transcription factors as potential substrates of the Hairy GRX.
Supervisor: Hudson, Andrew ; Oparka, Karl Sponsor: European Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: epidermal hairs ; trichomes ; Antirrhinum ; bald phenotype ; hairy phenotype ; genetic variation ; allelism tests