Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.

Population structure and genetic diversity in the nannandrous moss Homalothecium lutescens: does the dwarf male system facilitate gene flow?

Background

Nannandry is a sexual system where males (”dwarf males”) are much smaller than the conspecific females. Dwarf males occur in a wide range of unrelated organisms but the evolutionary advantages of this condition are poorly understood. The dwarf male sexual system results in differences in the mode of dispersal and establishment as well as the life span between males and females. Such differences must have profound effects on the population dynamics and genetic structures. We have studied four populations of the nannandrous moss Homalothecium lutescens in southern Sweden. We genotyped dwarf males and female shoots with the aim of describing the genetic diversity and structure of the populations.

Results

Dwarf males were most related to their host shoot, then their colony (within 0.5 m²) and then the rest of the population, which suggests restricted spore dispersal. However, a few dwarf males in each population appeared to originate from other colonies and sometimes even other populations. Genetic diversity of dwarf males was generally high but showed no tendency to be consistently higher or lower than female genetic diversity within the four populations.

Conclusions

Although most dwarf males have local origin, sporadic dispersal events occur. The ability of the dwarf males to establish in high numbers in mature colonies facilitates gene flow between populations as well as increases the potential to accumulate genetic diversity within populations.

Molecular anatomy of an onychophoran: compartmentalized sperm storage and heterogeneous paternity

Onychophorans (peripatus or velvet worms) show extraordinarily high local endemism, and cryptic species are common. As part of a programme addressing issues of endemicity at hierarchical spatial scales, we investigated reproduction in Euperipatoides rowelli (Onychophora: Peripatopsidae) using microsatellite analysis. This species is ovoviviparous, and females have up to 70 embryos in their uteri simultaneously. Batches of undeveloped and well-developed embryos may be present in the uteri of a female. Paired ovaries lead via a common oviduct into paired uteri, each of which has a spermatheca (sperm storage organ). Insemination in E. rowelli is dermal-haemocoelic: spermatophores are placed on the skin of the female, the body wall is breeched, and sperm are released into the haemocoel through which they migrate to the spermathecae. There is no obvious mechanism to prevent sperm mixing, yet microsatellite analysis indicated that offspring in a female's paired reproductive tracts can be sired by different males, and that the paired spermathecae can contain sperm from different males. More than 70% of females had broods with multiple paternity. The data are consistent with the potential for female postcopulatory influence over fertilizations: in particular, compartmentalization of sperm from different males into different spermathecae. Female control of fertilizations could lead to benefits including increased diversity of offspring, minimization of maternal-paternal genetic incompatibility, and influence on offspring genotypes. Multiple mating alone may increase the genetic diversity of offspring: this could be of importance in E. rowelli, which has very small genetic neighbourhoods and low genetic marker diversity.

Oogenesis in a placental viviparous onychophoran

This first ultrastructural study of oogenesis in a placental viviparous onychophoran describes oocyte differentiation, cell interactions and reveals various unusual cellular features. The viviparous onychophoran Plicatoperipatus jamaicensis has paired ovaries medially located, attached to the dorsal body wall by muscular terminal filaments. The rest of the female reproductive tract consists of paired spermathecae oviduct/uteri (hereafter referred to as uterus). Bulbous spermathecae are joined to the oviducts by ducts. Also continuous with the oviduct lumen are two tubular structures whose lumina open to the hemolymph. The uteri contain a progression of developmental stages from implantation through stalked morulae, blastocysts, larvae and juveniles about to be born. Growing oocytes are characterized by large germinal vesicles showing synaptonemal complexes. Oocytes are surrounded by flattened follicle cells that possess extensive bundles of thick and thin filaments. Mature oocytes contain little or no yolk, but are unique among organisms in accumulating a large central reservoir of stored glycogen. The lack of yolk reflects the placental viviparous nature of the reproductive process. The glycogen reservoir provides a rapidly accessible energy source for early developmental stages. Particularly prominent also are unusually extensive and highly elaborate Golgi complexes in the cortical and peri-nuclear ooplasm. While extensive Golgi complexes have been described in oocytes of a variety of species, the particularly exaggerated size and amount of Golgi in these onychophorans suggests they may provide excellent material for the study of Golgi function. The features of the oocyte and placental viviparity show this is an ideal model to investigate the nature of the placental reproductive process analogous to mammals in an invertebrate and its implications to oogenesis.