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.

SynopsisA genetic linkage map of guinea grass ( Panicum maximum Jacq.) was generated with nine of the AFLP markers found to be associated with apospory. These aposporyassociated markers were assigned to a linkage group having previous association with microsporogenesis of the aposporous guineagrass cultivar 'Natsukaze'. An aposporous linkage group was constructed utilizing 38 AFLP markers. Embryo sac analysis revealed that sexual and apomictic embryo sacs occurred at a frequency of 1 : 1, indicating simple inheritance of a single major gene controlling apospory in guineagrass. In addition, utilizing 56 AFLP primer combinations and 41 RAPD primers, 39 linkage groups and 360 simplex marker loci were assigned to the genetic map of the 'Natsukaze' cultivar. These markers covered 1703.5 cM of the autotetraploid guineagrass genome (2n = 4x = 32), with an average spacing of 4.7 cM. These tightly linked markers to apospory locus in guineagrass could be a powerful tool for marker-assisted selection of apospory and map-based cloning of the apospory gene.

Simple sequence repeats (SSR) have several positive attributes as genetic markers when compared with other DNA‐based marker systems. Because of their successful use for genetic analysis in a number of plant species, they could be similarly useful tools for the genetic study of various Zoysia species. A genomic library enriched for the AG/TC motif was developed from Zoysia japonica cv. ‘Asagake’ and screened by using 5′‐biotin‐labeled oligonucleotides (AG20). As a result of sequencing analysis of 162 clones, a total of 119 clones were identified containing 8–35 SSR repeats. Thirty‐two primer pairs designed from these clones were evaluated for their ability to detect polymorphisms among six additional zoysiagrass cultivars. The average values of the observed heterozygosities and polymorphic information content for the individual loci were 0.57 and 0.69, respectively. These primers were successful in generating several informative cross‐amplification products in additional Zoysia species. The generation of informative SSR markers should be a valuable tool for identification, estimation of genetic diversity and construction of genetic linkage maps in Zoysia species.

No abstract