The structure, meiotic behaviour and effects of B chromosomes in Briza humilis Bieb. (Gramineae)

Abstract

A single population of Briza humilis contained two types of B chromosome, one a large ( BL) and the other a small (Bs) acrocentric. DNA measurements show that the BL chromosome contains approximately twice as much DNA per unit length as the members of the regular complement. The meiotic pairing behaviour of the Bs is variable and BL and BS are seen to pair in some cells. The presence of BL depresses the chiasma frequency of the regular complement but the chiasma frequency of A and B chromosomes does not appear to be related. The transmission rate of the B chromosomes is variable and the BL shows a non-disjunction mechanism during microsporogenesis that is absent during megasporogenesis. For the BS chromosome the transmission rate is very low and there is no evidence of a non-disjunction mechanism. In general seeds containing BL chromosomes germinate more slowly than those without B chromosomes.

Evolution of B Chromosomes: From Dispensable Parasitic Chromosomes to Essential Genomic Players

“…The second model (White, 1973) proposed that B chromosomes may be maintained in the population without drive mechanisms if they have a beneficial effect on their carriers in small numbers but start to be detrimental in high copy numbers. Empirical data reviewed in this paper supports both models providing evidence for the selfish spread of B chromosomes in populations through drive in many species ( Hasegawa, 1934 ; Nur, 1962 ; Nur, 1963 ; Rutishauser and Rӧthlisberger, 1966 ; Nur, 1969 ; Kayano, 1971 ; Jones and Rees, 1982 ; Gregg at al., 1984 ; Murray, 1984 ; Viseras et al, 1990 ; Jones, 1991 ; Pardo et al, 1995 ; Houben, 2017 ; Jones, 2018 ; Clark and Kocher, 2019 ; Blavet et al, 2021 ) as well as identifying beneficial effects of B chromosomes for their hosts ( Williams, 1970 ; Dherawattana and Sadanaga, 1973 ; Rees and Hutchinson, 1974 ; Holmes and Bougourd, 1989 ; Miao et al, 1991 ; Plowman and Bougourd, 1994 ; Enkerli et al, 1997 ; Hatta et al, 2002 ; Nokkala et al, 2003 ; Rodriguez-Carres et al, 2008 ; Akagi et al, 2009 ; Coleman et al, 2009 ; Ma et al, 2010 ; Balesdent et al, 2013 ; Thatcher et al, 2016 ; Williams et al, 2016 ; Dalíková et al, 2017 ; Pereira et al, 2017 ; van Dam et al, 2017 ; Armitage et al, 2018 ; Kinsella et al, 2019 ; Torgasheva et al, 2019 ; Imarazene et al, 2021 ; Lewis et al, 2021 ). From the example in rye where B chromosomes have beneficial function but are still driving ( Pereira et al, 2017 ) we see that there could be even co-occurrence of drive and beneficial function which indicates the rather extensive complexity of B chromosome evolution.…”

Comprehending the dynamism of B chromosomes in their journey towards becoming unselfish

“…For a long time, Bs were considered genetically inert elements, utilizing replication machinery of the host like a parasite for their survival through drive ( Murray, 1984 ; Jones, 1991 ). They were thought to provide no advantage to their hosts and rather produced negative phenotypic effects and reduced fertility when present in high copy numbers ( Bougourd and Jones 1997 ; Camacho et al, 2004 ).…”

Are B chromosomes useful for crop improvement?

“…Ayonoadu and Rees (1968); Hanson (1969) Chiasma frequency reduced Aegilops speltoides, Ae. mutica (2n = 2x = 14 + 0-3Bs) Simchen, Zarchi, and Hillel (1971) Agropyron mongolicum, A. cristatum (2n = 2x = 14 + 0-6 Bs) Chen, Jahier, and Cauderon (1993) Artemisia frigida (2n = 2x = 18 + 0-2Bs) Bakshi, Kaul, and George (1987) Briza humilis (2n = 2x = 14 + 0 = 4Bs) Murray (1984) Lolium perenne (2n = 2x = 14 + 0-3Bs) Cameron and Rees (1967) Najas marina (2n = 2x = 12 + 1B) Viinikka (1973)…”