Translocation of a parthenogenesis gene candidate to an alternate carrier chromosome in apomictic Brachiaria humidicola

Abstract

BackgroundThe apomictic reproductive mode of Brachiaria (syn. Urochloa) forage species allows breeders to faithfully propagate heterozygous genotypes through seed over multiple generations. In Brachiaria, reproductive mode segregates as single dominant locus, the apospory-specific genomic region (ASGR). The AGSR has been mapped to an area of reduced recombination on Brachiaria decumbens chromosome 5. A primer pair designed within ASGR-BABY BOOM-like (BBML), the candidate gene for the parthenogenesis component of apomixis in Pennisetum squamulatum, was diagnostic for reproductive mode in the closely related species B. ruziziensis, B. brizantha, and B. decumbens. In this study, we used a mapping population of the distantly related commercial species B. humidicola to map the ASGR and test for conservation of ASGR-BBML sequences across Brachiaria species.ResultsDense genetic maps were constructed for the maternal and paternal genomes of a hexaploid (2n = 6x = 36) B. humidicola F1 mapping population (n = 102) using genotyping-by-sequencing, simple sequence repeat, amplified fragment length polymorphism, and transcriptome derived single nucleotide polymorphism markers. Comparative genomics with Setaria italica provided confirmation for x = 6 as the base chromosome number of B. humidicola. High resolution molecular karyotyping indicated that the six homologous chromosomes of the sexual female parent paired at random, whereas preferential pairing of subgenomes was observed in the apomictic male parent. Furthermore, evidence for compensated aneuploidy was found in the apomictic parent, with only five homologous linkage groups identified for chromosome 5 and seven homologous linkage groups of chromosome 6. The ASGR mapped to B. humidicola chromosome 1, a region syntenic with chromosomes 1 and 7 of S. italica. The ASGR-BBML specific PCR product cosegregated with the ASGR in the F1 mapping population, despite its location on a different carrier chromosome than B. decumbens.ConclusionsThe first dense molecular maps of B. humidicola provide strong support for cytogenetic evidence indicating a base chromosome number of six in this species. Furthermore, these results show conservation of the ASGR across the Paniceae in different chromosomal backgrounds and support postulation of the ASGR-BBML as candidate genes for the parthenogenesis component of apomixis.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-5392-4) contains supplementary material, which is available to authorized users.

Comparison of selection methods among and within full‐sibling progenies in Urochloa humidicola

“…Due to the expected segregation regarding apomixis in the progeny of crosses between apomictic and sexual parents (Valle & Savidan, ), when selecting individuals within progenies, their differentiation is necessary. For this, breeders may use morphological descriptors, anatomical analysis of embryonic sacs or, more recently, molecular markers (Bitencourt et al, ; Worthington et al, ; Zorzatto et al, ). Potentially superior apomictic hybrids can be directed by the breeder for clonal testing, as apomixis allows to fix additive and non‐additive genetic effects while fully preserving the heterosis manifested in the hybrid combination.…”

Apomixis Technology: Separating the Wheat from the Chaff

“…Genetic factors linked to apomixis components had been localized into large chromosomal regions of up to 50 Mb showing suppression of recombination [50,56] and hemizygous linkage groups (i.e., markers do not show hybridization signals in sexuals [56][57][58]). Apomixis loci of monocots and dicots allocate chromosome structural changes (mainly inversions and translocations; [50,58,59]) and share microsynteny among different apomictic and sexual species [60], a feature that may relay-at least in grasses-upon recombinogenic (euchromatic) or nonrecombinogenic (heterochromatic) regions [61]. Mechanisms operating to reduce recombination and retain linkage disequilibrium could be ascribed to allele sequence divergence, chromosomal rearrangements, and chromatin remodeling to enhance the formation of heterochromatin.…”

“…These studies represent a good progress toward understanding of parthenogenetic development. Moreover, the ASGR region and the PsASGR-BBML gene sequences are highly conserved across the Paniceae in Brachiaria and Panicum species having different chromosomal backgrounds, suggesting a relevant role for the parthenogenesis component of apomixis [59,134]. However, the variable penetrance of the trait between transgenic lines and siblings of the same line, the complexity of embryo development observed in rice lines with shifts to vivipary and absence of endosperm development [130] plus the lack of a reference genome of the apomictic species highly restrict the options to deeply characterize the PsASGR-BBML gene and the network of genes and/or protein interactions which may further promote parthenogenesis.…”

High-resolution Linkage Map with Allele Dosage Allows the Identification of an Apomixis Region and Complex Traits in Guinea Grass (Megathyrsus maximus)

“…Nonetheless, we mapped markers at a shorter distance (0.8 cM) from the apo-locus ( Figure 3). Genetic markers linked to apomixis have been sought in other forage grasses (Vigna et al, 2016;Worthington et al, 2016Worthington et al, , 2019, aiming at the efficient and rapid identification of the reproductive mode of progenies.…”