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Master of Science (MS)
Schwarz, Dietmar, 1974-
TTS (transmitting tissue-specific) proteins are abundant in the extracellular matrix (ECM) of the transmitting tissue, which forms the pollen tube pathway in Nicotiana pistils. These arabinogalactan proteins stimulate pollen tube growth and are vital for optimal seed set. I have cloned and sequenced two putative orthologs, PiPRP1 and PaPRP1, which are expressed in the pistils of Petunia integrifolia and Petunia axillaris, respectively. Comparison of the domain architecture and cross-reactivity with anti-TTS protein antibodies confirm that the proteins encoded by these Petunia cDNA clones are orthologs of TTS proteins (TTSPs) from Nicotiana species. Using immunological detection methods, I have shown that TTSP orthologs are present in the pistils of three subfamilies within the Solanaceae: Nicotianoideae, Petunioideae, and Solanoideae. Surprisingly, the proteins were also detected in leaves and roots of P. integrifolia seedlings. I cloned the TTSP ortholog expressed in seedling leaves (PiPRP2) and found it to be nearly indistinguishable from PiPRP1, encoded by the pistil cDNA. Like the TTSPs from Nicotiana, PaPRP1, PiPRP1, and PiPRP2 are histidine-domain arabinogalactan proteins with a highly variable proline-rich domain containing KPP repeats that vary in number and location among solanaceous taxa. Multiple alignments were used to deduce the effect of natural selection on the conserved and hypervariable domains of this multidomain subfamily of arabinogalactan proteins. For each pairwise comparison, I deduced the Ka/Ks ratio, which expresses the nucleotide substitutions per synonymous site (Ks) and non-synonymous site (Ka) in the two sequences. My analysis indicates that the two hypervariable domains of these proteins have undergone positive selection (Ka / Ks > 1), whereas the conserved domains are under purifying selection (Ka / Ks < 1). The differential selective pressure on the protein domains suggests that the hypervariable domains are involved in species-specific interactions with an unidentified pollen tube partner, and the conserved domains have general functions that are invariant. I propose that sequence divergence in the hypervariable domain reinforces speciation by generating a post-pollination prezygotic breeding barrier between incipient species.
Western Washington University
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Callaway, Tara D., "TTS protein orthologs as interspecific reproductive barriers in the solanaceae" (2012). WWU Graduate School Collection. 234.