Morphological and functional adaptations of the female reproductive system in Veigaiidae (Acari: Gamasida) and implications regarding the systematic position of the family

An overview of chelicerate ovaries, with special reference to mites - myths and facts

In arthropods of the subphylum Chelicerata a panoistic ovary, in which all germline cells differentiate into oocytes, prevails. Among the chelicerates, mites are believed to show a great variety of the structure of the female gonads. In general, the knowledge of the ovarian structure in mites is fragmentary and patchy. In both evolutionary lines, Acariformes and Parasitiformes, apart from the panoistic ovary, the meroistic ovary, in which the oocytes grow supported by their sibling cells, the nurse cells, occurs. The presence of the meroistic ovary is considered an apomorphic state. Previous studies revealed a various structure of the meroistic ovary in different mite taxa, and the differences came down, inter alia, to a different number and location of the nurse cells in relation to the oocytes. Here we provide a comprehensive review of the structure of the Chelicerata ovary, with special reference to the mite ovary. We also provide our preliminary results of the analysis of ovarian structure in two representatives of terrestrial Parasitengona (Acariformes), Allothrombium fuliginosum (Trombidiidae) and Erythraeus cinereus (Erythraeidae), performed using light, confocal and electron transmission microscopy. The analyses allowed for verification of data published before. In A. fuliginosum we showed the presence of the nurse cells in the ovarian wall, so the ovary should be classified as meroistic. In meroistic ovary of E. cinereus we found that each oocyte is connected to several mononucleated nurse cells. The verification of literature data and broadening the knowledge of the structure of the female gonad in mites, will result in estimating the usefulness of the ovary traits in phylogenetic analyses and will provide the basis for inference about the directions of evolutionary changes of female gonad at lower systematic levels.

On some morphological and ultrastructural features of the insemination system in five species of the genus Brevipalpus (Acari: Tenuipalpidae)

The genus Brevipalpus (Tenuipalpidae) includes 291 described species commonly found in the tropical and subtropical regions. Morphological characters considered in the taxonomy of Brevipalpus species are difficult to discern, which often leads to erroneous identifications and the presence of cryptic species within species is suspected. New morphological characters are now considered relevant for identification of Brevipalpus species; among them, the morphology of the seminal receptacle (spermatheca) of the female insemination system. This feature has not been considered relevant until now; thus, there is little information about the insemination system in the available species descriptions. Hence, in the present study, ultrastructural details are provided for the insemination system in five species of Brevipalpus, representing different morphological groups. The seminal receptacle (spermatheca) and the insemination duct are illustrated using light, transmission and scanning electron microscopy. The spermatheca proved to have specific morphological features that can be useful for taxonomic purposes. On the other hand, its appearance within a population might be variable in a way that needs to be ascertained and evaluated.

Ultrastructural and functional adaptations of the female reproductive system in the family Heterozerconidae (Acari, Anactinotrichida, Gamasida, Heterozerconina) and implications for the systematic position of the group

Heterozerconidae is a poorly known, early derived mite family belonging to Heterozerconina (Monogynaspida, Gamasida (= Mesostigmata)). The systematic position of the family is still controversial and little is known about the biology and anatomy of the taxon. In this paper, the gross anatomy, ultrastructure and functional morphology of the female reproductive system are described comparing genera from different geographic areas. The occurence of podospermy (i.e. the use of a sperm transfer process carried by the fixed digit of the male chelicerae to inseminate females through secondary insemination pores instead of through the oviporus) as insemination mode in this family was documented. Nevertheless, morphological and functional evidence in the reproductive system of the females supports the idea that, in the same family, more than one insemination mode is present: some genera are plesiomorphically tocospemic (i.e. insemination through the oviporus) while others switched to podospermy. Such discovery is of fundamental importance for the determination of the relationship between the family Heterozerconidae and the family Discozerconidae, both belonging tentatively to Heterozerconina and for the phylogenetic position of the Heterozerconina among Gamasida.

Gonads and gametogenesis in astigmatic mites (Acariformes: Astigmata)

Astigmatans are a large group of mites living in nearly every environment and exhibiting very diverse reproductive strategies. In spite of an uniform anatomical organization of their reproductive systems, gametogenesis in each sex is highly variable, leading to gamete formation showing many peculiar features and emphasizing the distinct position of Astigmata. This review summarizes the contemporary knowledge on the structure of ovaries and testes in astigmatic mites, the peculiarities of oogenesis and spermatogenesis, as well as provides new data on several species not studied previously. New questions are discussed and approaches for future studies are proposed.

Ultrastructure of the male genital tract, spermatogenesis and spermatozoa of Hattena cometis Domrow (Acari: Gamasida: Ameroseiidae)

The ameroseiid mite Hattena cometis has a male genital system that consists of an unpaired, u-shaped testis and paired deferent ducts leading into an unpaired accessory genital gland and ejaculatory duct. The genital opening is located anteriorly immediately in front of the sternal shield. Spermatogenesis is simple, probably due to the haploid nature of the male. Eight stages of spermatogenesis could be roughly distinguished. Mature spermatozoa as found in the deferent duct lumen are peculiar in having a bisected nucleus and numerous peripheral flat chambers, which were formed from indentations of the plasmalemma. In inseminated females, spermatozoa were observed in the syncytial tissue of the sperm access system and in the somatic cells of the ovary. These spermatozoa have achieved a new structure, i.e., an electron-dense plate dividing the cell into two unequal halves. The dense plate has an intricate substructure. Its function is unknown. These sperm cells are considered to represent capacitated spermatozoa. The peripheral chambers are reduced in number inside the female. Similar sperm cells, containing a dense plate, were seen in vacuoles within the epithelium of the deferent duct of one male. These cells are evidently under destruction, but before being completely dissolved had undergone a development leading beyond that of the mature sperm cells found in the deferent duct. Apparently, entering the cell of the deferent duct epithelium or the syncytium tissue triggers the production of the dense plate (or the capacitation process). Our observations are compared with results obtained from other anactinotrichid Acari, mainly Gamasida, and confirm and complete the interpretation of the correlated evolution of components of gamasid reproductive systems.