Isogamy

Abstract: Binary mating types are proposed to arise in a three-stage process through selection of nuclear genes to minimize cytoplasmic gene conflict at the time of gamete fusion. In support of this view we argue that: (i) in systems with fusion of gametes, the mating type genes are typically binary and regulate cytoplasmic inheritance; (ii) binary sexes have evolved several times independently associated with fusion, although at least twice binary types have been lost, associated with a loss of fusion; further, in accordance with the theory are findings for isogamous species that (iii) close inbreeding may correlate with less than two sexes and biparental inheritance of cytoplasmic genes; and (iv) species with more than two sexes may have uniparental inheritance of cytoplasmic genes, be rare and be afflicted by deleterious cytoplasmic genes which attempt to pervert normal cytoplasmic genetics. Such facts and their rationale support a new and unified definition of sexes based on the control of the inheritance of cytoplasmic genes. For the common cases, the male sex is that which resigns attempts to contribute cytoplasmic genes to the next generation. We differentiate between sexes and the incompatibility types of ciliates, basidiomycetes, some angiosperms and a few other organisms which are independent of organelle contribution.

Abstract: Water striders (Heteroptera: Gerridae) exhibit two different types of mating behavior. The most common mating system (type I) is characterized by strong apparent conflicts of interest between the sexes, and conspicuous pre– and postcopulatory struggles. Some species exhibit a mating system that involves much less apparent conflict (type II) and lack the intense copulatory struggles. I argue that the predominant mating system in water striders is a direct consequence of sexual conflicts over mating decisions. Matings involve high costs to females (increased predation risk and energetic expenditure) but few, if any, balancing direct benefits. Sperm–displacement rates are high, and males thus gain from rematings. Mating frequencies are high; females mate multiply for reasons of convenience. In these species, males are considered to have ‘won’ the evolutionary conflict over the mating decision in the sense that they have made acceptance of superfluous matings ‘the best of a bad job’ for females, by evolving behavioral and morphological traits that make it costly for females to reject males attempting copulations. Females, however, have apparently evolved a variety of counteradaptations to male harassment, to gain control over mating. Further, I suggest that sexual conflict may have played a crucial role in the evolution of type II matings from type I matings.

Abstract: Maternal inheritance of chloroplast genes occurs in the isogamous green alga Chlamydomonas reinhardii1,2. It has been shown3–5 using biochemical techniques that the chloroplast DNA of male origin is preferentially lost by 6 h after mating. DNAs in the chloroplast are organized by proteins into about 10 chloroplast nucleoids6,7. Therefore, if chloroplast DNA in zygotes is preferentially destroyed, the disappearance of chloroplast nucleoids from male gametes should be observable during zygote formation by high resolution epifluorescent microscopy. Here we present the first fluorescent microscopic evidence that in C. reinhardii, about eight chloroplast nucleoids from the male parent disappear during the first 40–50 min after mating, while those from the female parent persist and finally fuse together to form one large chloroplast nucleoid.

Abstract: Clones of the domoic-acid-producing pennate diatom Pseudo-nitzschia multiseries (Hasle) Hasle and of the potentially toxic P. pseudodelicatissima (Hasle) Hasle normally decrease in cell size in culture until they eventually die without undergoing sexual reproduction to regain the largest cell size. However, we induced sexual reproduction by mixing individual exponentially growing clonal cultures of the appropriate minimal cell size under the same conditions that are normal for vegetative growth. We observed pairing of parent cells (gametangiogamy); production of four morphologically isogamous, nonflagellated gametes per gametangial pair; rearrangement of the gametes and their fusion to form zygotes, revealing physiological anisogamy; enlargement of auxospores; and formation of long initial cells. Our observations of allogamous reproduction are consistent with those reported for other dioecious pennate diatoms. Clones of P. pseudodelicatissima from the Black Sea and from the CCMP culture collection failed to auxosporulate when mixed together, although they are the same species according to scanning electron microscopy. The range in apical length of P. multiseries was broader than that reported in the literature for field samples, necessitating a modification of the species description. Knowledge of the pattern and timing of sexual reproduction in Pseudo-nitzschia spp. may provide insights into their bloom dynamics.