Tripsacum dactyloides (Poaceae): a natural model system to study parthenogenesis

Abstract

Diplosporous apomeiosis, formation of unreduced embryo sacs primarily of the Antennaria type, followed by parthenogenetic embryo development and pseudogamy (fertilization of the central cell) describe gametophytic apomixis within the Tripsacum agamic complex. Tripsacum dactyloides (Eastern gamagrass) is a close relative of domesticated maize and was chosen as a natural model system to investigate gene expression patterns associated with parthenogenesis. The genome size of diploid sexual and polyploid apomictic T. dactyloides was estimated by flow cytometry to be 7.37 pg (2C), 14.74 pg (4C) and 22.39 pg (6C), respectively. The diploid genome size is thus approximately 1.35× larger than that of maize. The apomeiotic-pseudogamous pathway of seed formation was demonstrated at a rate of 92% by the flow cytometric seed screen (FCSS) with single mature seeds in tetraploid accessions. This number includes twin embryos which were detected in 13% of the seeds analyzed. Fertilization of unreduced egg cells (BIII hybrids) was measured in 10% of apomictic seeds. Autonomous (fertilization-independent) embryo development and fertilization-dependent endosperm formation were confirmed by pollination of tetraploid T. dactyloides with a diploid transgenic maize line carrying an actin:: β -glucuronidase (GUS) reporter construct. GUS expression was detected after pollination in the developing endosperm, but not in the embryo. In similar intraspecific crossing experiments with maize, GUS expression was detected in both the embryo and endosperm. A protocol was established for microdissection of embryo sacs and early parthenogenetic embryos of T. dactyloides. Together, these techniques provide new tools for future studies aimed at comparing gene expression patterns between sexual maize and sexual or apomictic T. dactyloides.

Seed development and inheritance studies in apomictic maize-Tripsacum hybrids reveal barriers for the transfer of apomixis into sexual crops

“…Further differentiation into megagametophytes resembles that of the Polygonum type found in sexual species, but cells contain nuclei of maternal genetic makeup. Activation of unreduced egg cells through unknown developmental alterations in female gametogenesis induces embryogenesis in absence of fertilization (Bantin et al, 2001, Farquharson, 1955 but the developmental course of maternal embryos stops after a few rounds of mitotic divisions resulting in quiescent proembryos within unfertilized megagametophytes (Grimanelli et al, 2003). Pollination, followed by the delivery of two sperm cells into the mature megagametophyte and by the fertilization of the central cell only, is required for seed development.…”

Studying allozyme variation in sexual and apomictic Taraxacum and Pilosella (Asteraceae) populations

“…However, the use of the modern methods of research (molecular techniques and gene engineering) in combination with the traditional approaches (hybridological, cytoembryological, and cytogenetic) did not provide answers to most questions related to this problem. For instance, the explanations of the genetic control of this phenomenon are still hypothetical, although some progress has been made in this direction [1][2][3][4][5]. The character and nature of genetic variability in apomictic populations [6,7] and the role of gametophytic apomixis in evolution remain largely unclear.…”

Genomic Resources for Gene Discovery, Functional Genome Annotation, and Evolutionary Studies of Maize and Its Close Relatives

“…It represents an untapped genetic resource for crop improvement and contains many useful agronomic traits like disease and pest resistance and harsh-environment adaptation (Hoisington et al 1999;Chia et al 2012). It is also a natural model system to study parthenogenesis, an important trait for crop improvement (Hoisington et al 1999;Bantin et al 2001). Tripsacum can hybridize with maize and the hybrids can grow well (Harlan and de Wet 1977).…”