Abstracts - Society for Developmental Biology

4
Program/Abstract # 10
Evolution of sex determination in animals that produce males, females and hermaphrodites
da Silva, André Pires; Kache, Vikas, University of Texas, Arlington, United States; von Reuss, Stephan (Cornell
University, Ithaca, United States); Chaudhuri, Jyotiska; Bateson, Christine (U Texas, Arlington, United States)
The evolution of mating systems has fascinated biologists since the time of Darwin, specifically the causes and
consequences of a species transition from one mating system (e.g. dioecy) to another (e.g. hermaphroditism). Theory
predicts that these transitions likely involve one or more intermediates. To understand how animals transition from one
mating system to another, we are studying the mechanisms by which a yet undescribed nematode generates male, female
and hermaphrodite progeny. This is likely to represent a transitory system from the ancestral male/female to a
hermaphroditic mode of reproduction.We found that the male /non-male decision in this nematode is chromosomally
determined, whereas the hermaphrodite/female decision is epigenetic. We identified the molecular nature of a pheromone
that alters the development of female-fated juveniles to develop into hermaphrodites. The study of the molecular
mechanisms underlying sex determination in this and other closely species might shed some light in how mating systems
evolve.
Program/Abstract # 11
Unraveling a transcriptional network involved in maize domestication
Doebley, John, University of Wisconsin, Madison, United States
Maize is a domesticated form of a wild Mexican grass called teosinte. The domestication of maize from teosinte occurred
about 9,000 years ago. As a result of human (artificial) selection during the domestication process, dramatic changes in
morphology arose such that maize no longer closely resembles its teosinte ancestor in ear and plant architecture. We have
identified and analyzed three of the genes involved in these morphological changes. First, teosinte branched (tb1) is
largely responsible for the difference between the long branches of teosinte versus the short branches of maize. tb1 encodes
a transcriptional regulator that functions as a repressor of branch elongation. Gene expression analysis indicates that the
product of the teosinte allele of tb1 accumulates at about half the level of the maize allele. Fine-mapping experiments show
that the differences in phenotype and gene expression are controlled in part by an upstream transposon insertion that acts
as an enhancer of gene expression. Second, teosinte glume architecture (tga1) is largely responsible for the formation of a
casing that surrounds teosinte seeds but is lacking in maize. tga1 also encodes a transcriptional regulator, however in this
case a single amino acid change represents the functional difference between maize and teosinte. This single amino acid
change appears to convert the maize allele into a transcriptional repressor of target genes. Third, grassy tillers (gt1)
contributes to differences in plant architecture and encodes an HD-ZIP transcription factor. Causative changes at gt1
appear to be complex, involving multiple changes. tb1, tga1 and gt1 are members of the same developmental network
which regulates shade avoidance. This pathway was a target of human selection during the domestication process.
Program/Abstract # 12
Evolution of obligate heterodimerization among grass B class genes
Whipple, Clinton J.; Bartlett, Madelaine; Williams, Steven, Brigham Young University, Provo, United States
Homeotic regulation of floral organ identity, as described in the ABC model of floral development, is primarily controlled
by MADS-box transcription factors. B class genes regulate second (petal) and third (stamen) whorl identities, and are
represented by two paralogous gene lineages, DEF/AP3 and GLO/PI. Unlike other ABC MADS box proteins that can bind
DNA as homodimers, B class proteins only bind DNA as obligate heterodimers. B class genes also undergo positive
autoregulation, and it has been proposed that B class obligate heterodimerization evolved from sequential loss of the
ability to homodimerize and that the unique combination of obligate heterodimerization with an autoregulatory feedback
mechanism was important in the canalization of eudicot flowers. We have been examining B class function in the monocot
model grass species maize (Zea mays). We have positionally cloned the sterile tassel silky ear1 (sts1), which has a
phenotype similar to other B class mutants in the grass family. sts1 encodes one of three GLO/PI orthologs in maize, but
shows no evidence of functional redundancy as it regulates transcription of the paralogous Zmm18 and Zmm29.
Interestingly, maize B class proteins bind DNA as an obligate heterodimer, while B class proteins from close outgroups
homodimerize. In order to understand the evolution of obligate heterodimerization in the grasses, we have isolated GLO/PI
orthologs from diverse grasses and outgroups and assayed their dimerization specificity. Our results indicate that shifts to
and from obligate heterodimerization are frequent, and controlled by a small number of amino acid changes. The possible
developmental consequences of this evolution in protein binding will be discussed.