Academic journal article Genetics

Identification of the Minus-Dominance Gene Ortholog in the Mating-Type Locus of Gonium Pectorale

Academic journal article Genetics

Identification of the Minus-Dominance Gene Ortholog in the Mating-Type Locus of Gonium Pectorale

Article excerpt

ABSTRACT

The evolution of anisogamy/oogamy in the colonial Volvocales might have occurred in an ancestral isogamous colonial organism like Gonium pectorale. The unicellular, close relative Chlamydomonas reinhardtii has a mating-type (MT) locus harboring several mating-type-specific genes, including one involved in mating-type determination and another involved in the function of the tubular mating structure in only one of the two isogametes. In this study, as the first step in identifying the G. pectorale MT locus, we isolated from G. pectorale the ortholog of the C. reinhardtii mating-type-determining minus-dominance (CrMID) gene, which is localized only in the MT^sup -^ locus. 3'- and 5'-RACE RT-PCR using degenerate primers identified a CrMID-orthologous 164-amino-acid coding gene (GpMID) containing a leucine-zipper RWP-RK domain near the C-terminal, as is the case with CrMID. Genomic Southern blot analysis showed that GpMID was coded only in the minus strain of G. pectorale. RT-PCR revealed that GpMID expression increased during nitrogen starvation. Analysis of F^sub 1^ progeny suggested that GpMID and isopropylmalate dehydratase LEU1S are tightly linked, suggesting that they are harbored in a chromosomal region under recombinational suppression that is comparable to the C. reinhardtii MT locus. However, two other genes present in the C. reinhardtii MT locus are not linked to the G. pectorale LEU1S/MID, suggesting that the gene content of the volvocalean MT loci is not static over time. Inheritance of chloroplast and mitochondria genomes in G. pectorale is uniparental from the plus and minus parents, respectively, as is also the case in C. reinhardtii.

OOGAMOUS reproduction, which involves anisogamous fusion of distinctive sperm and egg cells, has apparently evolved from isogamous sexual reproduction where gametes of different mating types are very similar in size and appearance. Although oogamy is known in animals and land plants, the origins of oogamy are so ancient that there seem to be no extant isogamous close relatives of them (Karol et al. 2001; Rokas et al. 2005). The volvocine or colonial volvocalean algae are amodel lineage for studying the evolution of sexual reproduction for two reasons: first, they have both isogamous (Gonium, Pandorina, and Yamagishiella) and anisogamous/oogamous (Eudorina, Pleodorina, and Volvox) genera, the latter forming bundles of male gametes (sperm) and large female gametes (eggs), which are phylogenetically well studied (Nozaki and Itoh 1994; Nozaki et al. 2000); second, several mating-typespecific genes have been identified in the closely related isogamous, unicellular alga Chlamydomonas reinhardtii (Ferris et al. 1995; Ferris and Goodenough 1997). Therefore, the volvocine algae possess unrivaled features for studying the evolution of sex in terms of molecular biology (Kirk 2005).

Gonium pectorale has flattened 16-celled colonies and produces isogametes in sexual reproduction. Heterothallic sexuality in G. pectorale with two mating types, plus and minus, was studied by Schreiber (1925) and Stein (1958). Although only one (plus) of the two conjugating isogametes of C. reinhardtii has a tubular mating structure (TMS), both isogametes of G. pectorale extend a TMS toward the other (Nozaki 1984; Nozaki and Itoh 1994). Phylogenetic analyses imply that anisogamous/ oogamous species of the colonial Volvocales evolved from an ancestral colonial species that exhibits isogamy as in G. pectorale (Nozaki and Itoh 1994; Nozaki et al. 2000). The genus Gonium is phylogenetically important, as it represents the most basal lineage within the relatively advanced volvocine algae composed of isogamous genera and anisogamous/oogamous members (Nozaki et al. 2000). In C. reinhardtii the mating-type (MT) loci-mating type plus (MT^sup +^) and mating type minus (MT^sup -^)-consist of a 200-kb region dimorphic between the two mating types, referred to as the rearranged (R) domain, and include genes involved in mating-type determination and in the function of the TMS, as well as housekeeping genes with alleles present in both MT loci. …

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