The Levels of Male Gametic Mitochondrial DNA Are Highly Regulated in Angiosperms with Regard to Mitochondrial Inheritance

Abstract

The mechanisms that regulate mitochondrial inheritance are not yet clear, even though it is 100 years since the first description of non-Mendelian genetics. Here, we quantified the copy numbers of mitochondrial DNA (mtDNA) in the gametic cells of angiosperm species. We demonstrate that each egg cell from Arabidopsis thaliana, Antirrhinum majus, and Nicotiana tabacum possesses 59.0, 42.7, and 73.0 copies of mtDNA on average, respectively. These values are equivalent to those in Arabidopsis mesophyll cells, at 61.7 copies per cell. On the other hand, sperm or generative cells from Arabidopsis, A. majus, and N. tabacum possess minor amounts of mtDNA, at 0.083, 0.47, and 1 copy on average, respectively. We further reveal a 50-fold degradation of mtDNA during pollen development in A. majus. In contrast, markedly high levels of mtDNA are found in the male gametic cells of Cucumis melo and Pelargonium zonale (1296.3 and 256.7 copies, respectively). Our results provide direct evidence for mitochondrial genomic insufficiency in the eggs and somatic cells and indicate that a male gamete of an angiosperm may possess mtDNA at concentrations as high as 21-fold (C. melo) or as low as 0.1% (Arabidopsis) of the levels in somatic cells. These observations reveal the existence of a strong regulatory system for the male gametic mtDNA levels in angiosperms with regard to mitochondrial inheritance.

A Conserved, Mg2+-Dependent Exonuclease Degrades Organelle DNA duringArabidopsisPollen Development

“…As expected, ptDNAs and mtDNAs had significantly increased in dpd1-1 compared with Col (n = 3, Welch's t test, P = 0.0041 and 0.0226, respectively) ( Figures 5A and 5B). A large difference in mtDNA levels was detected, probably because of an extremely low level of mtDNA in Arabidopsis wild-type pollen grains (Wang et al, 2010). By contrast, young seedlings showed no significant differences between Col and dpd1-1 in the level of ptDNA and mtDNA (P = 0.2887 and 0.2692, respectively) (Figures 5C and 5D).…”

“…When crossed with ecotype Cape Verde Islands (Cvi), with polymorphic ptDNA (Martínez et al, 1997), dpd1 (Col background) showed normal maternal inheritance of the plastid genome, as did the wild type (n = 65; see Supplemental Figure 6 online). Plastids have not been detected inside Arabidopsis sperm cells (Tang et al, 2009;Wang et al, 2010). Therefore, exclusion of plastids from the male gamete is perhaps the dominant mechanism for plastid maternal inheritance in Arabidopsis.…”

“…More importantly, some mitochondria might lack a complete genome (Preuten et al, 2010). A decrease in mtDNAs has also been reported during pollen development (Wang et al, 2010). These circumstantial observations prompted us to study organelle DNA degradation by a forward genetic approach.…”

“…Irrespective of the inheritance mode, however, we noticed that organelle DNAs are cytologically absent in pollen vegetative cells (Matsushima et al, 2008a;Sakamoto et al, 2008). A survey of numerous mature pollen grains using DAPI staining has revealed that almost all angiosperm species lack cytologically detectable organelle DNAs in pollen vegetative cells (Corriveau and Coleman, 1988;Mogensen, 1996;Zhang et al, 2003;Wang et al, 2010). It is noteworthy that pollen vegetative cells do not contribute to fertilization, but they do contain numerous plastids and mitochondria, which might be necessary for a pollen tube to germinate, elongate, and deliver sperm cells into the embryo sac.…”

Tissue-specific organelle DNA degradation mediated by DPD1 exonuclease

“…For example, multiple copies of mitochondrial DNA are dispensable for mitochondrial function in vegetative and reproductive organs. 6,7 Under phosphate-limiting conditions, chloroplast DNA was reportedly degraded in Chlamydomonas, possibly as a compensation mechanism for survival. 8 Furthermore, the characterization of plastidial uracil phosphoribosyltransferases showed that plastid functions as the…”