Brevipalpus yothersi, Baker, 1949

Sinico, Thaís Elise, Nunes, Maria Andréia, Kitajima, Elliot Watanabe, Cunha, Bruna Aparecida & Novelli, Valdenice Moreira, 2022, Notes on the embryological development of the Brevipalpus yothersi (Acari: Tenuipalpidae), Acarologia 62 (1), pp. 113-119 : 114-116

Brevipalpus yothersi embryological development

The B. yothersi embryological development was observed for seven days under experimental conditions and the eggs were easily visible by LM ( Figure 1 ). Eggs collected soon after oviposition were slightly elongated and elliptical in shape (average of 108 μm length x 70 μm

width) ( Haramoto 1969), with light orange coloration, and nuclei that appeared red.

At the earliest cleavage stages, about one-hour post-laying, the nucleus occupied the center of the egg ( Figure 1b ). Two hours after oviposition, the first division began with the separation of the nucleus into two blastomeres of the same size and shape ( Figure 1c ) divided by a thin membrane. At three hours post-oviposition, an equal cleavage resulted in the total division of blastomeres into four cells ( Figure 1d ). Five hours after, two new divisions occurred with the formation of the eight and 16-cells stages ( Figure 1e, f ). After, a large number of cells were observed, suggesting the occurrence of three more cleavages, reaching a total of about 128

blastomeres ( Figure 1g ), following the formation of a cluster of cells, the morula.

In a period of 20 to 24 hours after egg-laying, the cells migrated and agglomerated in the peripheral region of the egg forming the blastula ( Figure 1h ). After 48 hours, the formation of the germinal disc (periblastula) in the ventral region was observed ( Figure 1i ). The periblastula started to stretch and then becomes flattened ( Figure 1j ). After three days, the embryo was laterally inside the egg, initiating the formation of the primitive body ( Figure 1k ), where it was visible the formation of metamers ( Figure 1l ). From the fourth to the fifth day, the formation of primordial appendages was evident ( Figure 1m ). On the sixth post-oviposition day, the dark-colored ocelli formation was initiated ( Figure 1n ), being more visible on the seventh day before the larvae hatched ( Figure 1o ), and ending with hatching of a hexapod larvae ( Figure 1p ).

This pattern is according to other arachnids that only acquire the fourth pair of legs after their larvae undergo moults for the subsequent developmental stage ( Dearden et al. 2002 ; Sharma

2018).

The analysis of the B. yothersi eggs in LM allowed the description of the different embryological stages. To our best knowledge, this report is the first embryological description of false spider mites and demonstrated that the use of the liquid paraffin was essential to observe stage;

– Two-cell stage embryo divided by visible membrane (arrow); (d) – Second cleavage: four-cell stage; (e) – Eight-cell stage; (f) – 16-cell

(g) – 128-cell stage; (h) – Migration of cells to the peripheral region: blastula formation; (i) – Visible germinal disc in the ventral region; (j) – Distension of the germinal disc on the side of the egg; (k) – Primitive body formation; (l) – Formation of metamers (arrow); (m) – Primordial appendages (arrow); (n) – Initiation of ocelli formation, more evident before (o) – hatching of the egg with emerging larva; (p) — Hexapod larvae seven days after oviposition. Scale bar length (a) 100 µm (magnification of 20x) and (b-p) 50 µm (magnification of 40x).

the first hours of the false spider mite development, since it enables us to monitor the sequence of embryonic divisions and to establish the exact period it occurred.

The eggs structure of the B. yothersi is a centrolecithal type, where the nucleus occupies a central position ( Figure 1b ), as described Ornithodoros in moubata Murray, an Argasidae tick (Aeschlimann and Hess 1984). Here, the cleavage observed was a superficial type, similar to descriptions made on Prostigmata mites ( Gotoh et al. 1994 ; Dearden et al. 2002), but differs from previous proposals that holoblastic cleavage was a standard feature of Acari

( Laumann et al. 2010b). However, embryological studies of the Eriophyoidea superfamily have reported that their cleavage is meroblastic type, with the nuclei founded in a single region,

whereas the other regions of the egg contains only the yolk ( Chetverikov and Desnitskiy 2016).

These authors observed that nucleus division occurs at the egg periphery, a peculiarity of the embryological development of the Eriophyoidea family and differing from the other organisms of the Acari group. It particularly raises a possible hypothesis that a lineage of eriophyid mites could have diverged from the primitive Acari and transitioned from a superficial cleavage to a total cleavage and thereby acquiring it unusual meroblastic pattern.

The first cleavages of B. yothersi were similar to the mite species of the Panonychus genus

( Tetranychidae ) as reported by Gotoh et al. (1994). As illustrated and described in the 18

stages, noting that the interval between the first cell divisions was about one hour each and that from 100 blastomeres it was difficult to distinguish them. After observation of the cellular stage composed of 128-cells, the formation of the morula occurred, as visualized in B. yothersi .

The development of the B. yothersi embryos was slow compared to the tenuipalpid

Dolichotetranychus coco Flechtmann & Fernando , maintained at a temperature of 25 °C ± 1

°C, where the formation of the first pair of legs and morphogenesis occurred after 35 and 47

hours, respectively ( Santhosh et al. 2009). Here, the initial formation of the primitive body occurred on the third day after oviposition. Studies have reported that temperature variations and host plants affect the life cycle and embryonic development of different species of the mites ( Haramoto 1969 ; Chiavegato 1986 ; Teodoro and Reis 2006 ; Santhosh et al. 2009).

Thus, to avoid any possible influence, during egg analysis, the environmental conditions were controlled.

The chelicerates have some conserved characteristics during embryonic development. First,

the prosoma is emerged followed by the development of the appendices, which appear as small buds of tissue ( Sharma 2018) observed between the third and fifth days of the development of

B. yothersi embryos. Other conserved characteristics were the migration from the mouth to a terminal region below the chelicerae and the formation of the ocelli ( Sharma 2018). The total development time observed in B. yothersi was seven days (from the first cleavage to larvae hatching), differing from that observed in the T. urticae model mite, which has a complete embryonic development in 39 hours (25 °C ± 1 °C) and completing the life cycle in less than seven days ( Rao et al. 1996 ; Dearden et al. 2002 ; Grbic et al. 2007 ; Khila and Grbic 2007).

Figures

Figure 1 Development timeline of the Brevipalpus yothersi: (a) – B. yothersi female; (b) – Uncleaved egg: nucleus in the center of the egg; (c)