Abstract Detail

Macroevolution

Taylor, Anthony [1], Sheltra, Matthew [2], Huff, David [3].

he Buffalograss-Salmacisia pathosystem as a potential driving force for dioecious host sex evolution.

The parasitic fungus Salmacisia buchloëana induces ovary development in genetically male plants of its host, buffalograss (Bouteloua dactyloides), begging the question of whether the parasite has played a role in the host’s evolution of dioecy. Here, we argue for the involvement of Salmacisia in the evolution of separate sexes in buffalograss. Buffalograss serves as the definition of dioecy among North American perennial grasses, although it likely evolved from a hermaphroditic ancestor. Salmacisia is an endoparasitic fungus that lives intercellularly within the meristematic tissue of infected plants, and it is likely that the two species have a long coevolutionary history, with Salmacisia’s ancestor presumably causing a similar bunt disease in buffalograss’ hermaphroditic ancestor. Salmacisia infection causes bunt disease by replacing the contents of host ovaries, both induced and natural, with teliospores. Salmacisia infection also increases the number of flowers per plant by a factor of 26-fold in females and 2.4-fold in males. We hypothesize that in the buffalograss-Salmacisia pathosystem, co-evolution is occurring following the 'guard hypothesis,' a version of H.H. Flor’s ‘gene-for-gene’ hypothesis whereby the host’s resistance (R) genes detect the pathogen’s avirulence (Avr) genes through indirect interaction. We propose that Salmacisia drove the evolution of sexually dimorphic dioecy in buffalograss through an androdioecious pathway. We speculate that this occurred as a byproduct of buffalograss’ ETI against Salmacisia effectors, following the indirect interaction model (the ‘guard’ hypothesis). Our hypothesized candidate for a putative R gene involved in this interaction is one coding for a protein product with a structure similar to the BRI-associated kinase 1 (BAK-1) protein involved in brassinosteroid signaling in PAMP-triggered immunity. This kinase (BAKL) would function to sense the gibberellins modified by Salmacisia in an attempt to lower host defense responses. This would subsequently lead to an accumulation of brassinosteroid in the local tissue, which the buffalograss homolog of maize TASSELSEED2, known to cause selective cell death via the accumulation of its gene products, would use as a substrate. This would lead to apoptosis of the tissue the brassinosteroids were accumulating in: the pistil. Because male function would likely not be affected in the ancestral hermaphrodites where this occurred, plants with this novel BAKL allele could breed with the uninfected hermaphrodites in the population and spread the allele. If these males were significantly fitter than hermaphrodite males, e.g., by avoiding inbreeding depression, then directional selection would favor variants of this BAKL allele that result in pistil abortion even in uninfected plants. With higher male-male competition, feminized hermaphrodites would now have an advantage, and feminizing genes would be selected for, eventually leading to a novel male suppressor gene and unisexual females. Since the Salmacisia genome has just been sequenced, there are no proposed candidates for the fungal effectors involved.

1 - 745 Stratford Drive, Apt 745, State College, PA, 16801, United States
2 - Penn State University, Huck Institutes for the Life Sciences, Plant Biology, 101 Huck Life Sciences Building, University Park, Pennsylvania, 16802
3 - Penn State University, Plant Sciences, 103 Tyson Bldg, State College, PA, 16802, USA

Keywords:
Dioecy
Evolution
Sex
Parastite
Parasitic
Buffalograss
Fungi
Fungus
Host-parasite
Red Queen Hypothesis
Dioecious
Induced Hermaphroditism.

Presentation Type: Oral Paper
Number: MACRO II001
Abstract ID:220
Candidate for Awards:None