Genomic Constitution and Relationship in Urochloa (Poaceae) Species and Hybrids

Abstract

Urochloa P. Beauv. [syn. Brachiaria (Trin.) Griseb.] belongs to the family Poaceae and comprises species presenting different ploidy levels and modes of reproduction. Specific studies on genomic constitution and relationship within the genus Urochloa are limited, despite the great economic and agronomic importance of forage species and the high frequency of polyploids. Therefore, molecular cytogenetic analyses can enhance the knowledge about genomes and their differentiation, which allows making inferences about phylogenetic and genomic relationships. The current study aimed to investigate the genomic relationships between Urochloa ruziziensis (R. Germ. & Evrard) Crins (syn. Brachiaria ruziziensis Germain & Evrard.) cultivar ‘Kennedy’, Urochloa decumbens (Stapf) R. D. Webster [syn. Brachiaria decumbens (Stapf)] cultivar ‘Basilisk’, Urochloa brizantha (Hochst. ex. A. Rich.) R. D. Webster [syn. Brachiaria brizantha (A. Rich.) Stapf] cultivar ‘Marandu’, and their interspecific hybrids by means of genomic in situ hybridization, fluorescent in situ hybridization, and nuclear genome quantification by flow cytometry. We proposed the genomic constitution of U. ruziziensis, U. decumbens, and U. brizantha, such as B2B2, B1B1B2B2, and BBB1B1, respectively. Genomes B, B1, and B2 were considered to be homoeologous, but there was less affinity between genomes B and B2.

An Overview of the Genetics and Genomics of the Urochloa Species Most Commonly Used in Pastures

“…These studies have revealed several abnormalities typical of allopolyploids and segmental allopolyploids, such as asynchronous chromosome segregation, intragenomic pairing, and the presence of multivalents and micronuclei. More recently, de Paula et al (2017) confirmed the allopolyploidy of U. decumbens 4x and U. brizanth a 4x and Corrêa et al (2020) observed that the diploid genotypes of these species are potential ancestors of allotetraploids. Both studies were performed via genomic in situ hybridization (GISH).…”

“…Based on genomic in situ hybridization, de Paula et al (2017) recently suggested the allopolyploid origin of U. brizantha and U. decumbens , and such results were validated by additional GISH analysis ( Corrêa et al, 2020 ). The authors proposed genomic compositions of BBB 1 B 1 for U. brizantha , B 1 B 1 B 2 B 2 for U. decumbens , and B 2 B 2 for the diploid U. ruziziensis .…”

“…The Embrapa collection has been evaluated in terms of morphological and agronomic characteristics, reproductive mode (apomictic or sexual), cytogenetics, and genetic diversity, this one mainly involving the use of molecular markers. (e.g., Valle and Savidan, 1996 ; Mendes-Bonato et al, 2002a , b ; Boldrini et al, 2006 ; Risso-Pascotto et al, 2006 ; Nielen et al, 2009 ; Ambiel et al, 2010 ; Jungmann et al, 2010 ; Azevedo et al, 2011 ; Boldrini et al, 2011a ; Vigna et al, 2011a ; Garcia et al, 2013 ; Pessoa-Filho et al, 2015 ; Santos et al, 2015a ; de Paula et al, 2017 ; Souza et al, 2018 ; Corrêa et al, 2020 ; Moraes et al, 2021 ). Based on morphological and agronomic traits, U. brizantha exhibited the greatest variation, which reflects the larger number of accessions and the greater inherent genetic variability.…”

Association Mapping Considering Allele Dosage: An Example of Forage Traits in an Interspecific Segmental Allotetraploid Urochloa spp. Panel

“…Genome-wide association studies for U. brizantha and U. decumbens are even harder due to the species being segmental allotetraploids, one with genome allotetraploid behavior and the other with autotetraploid behavior (Worthington et al, 2016;de Paula et al, 2017). These species have chromosomes with preferential pairing and fully homologous chromosomes pairing at random (Sybenga, 1996).…”

“…In this case, genomic studies can be performed using markers with diploid and tetraploid allele dosage. The genomic constitution of U. ruziziensis, U. decumbens, and U. brizantha is classified as B 2 B 2 , B 1 B 1 B 2 B 2 , and BBB 1 B 1 , respectively (de Paula et al, 2017). However, although these genomes have been considered homeologous, less affinity is observed between genomes B and B 2 , and thus genes segregating as a diploid is possible in hybrids between U. ruziziensis and U. brizantha.…”