Chromalveolata

However, the monophyly of the Chromalveolata has been rejected. Thus, two papers published in 2008 have phylogenetic trees in which the chromalveolates are split up, and recent studies continue to support this view.

Groups and classification

Historically, many chromalveolates were considered plants, because of their cell walls, photosynthetic ability, and in some cases their morphological resemblance to the land plants (Embryophyta). However, when the five-kingdom system (proposed in 1969) took prevalence over the animal–plant dichotomy, most of what we now call chromalveolates were put into the kingdom Protista, but the water molds and slime nets were put into the kingdom Fungi, while the brown algae stayed in the plant kingdom. These various organisms were later grouped together and given the name Chromalveolata by Cavalier-Smith. He believed them to be a monophyletic group, but this is not the case.

In 2005, in a classification reflecting the consensus at the time, the Chromalveolata were regarded as one of the six major clades of eukaryotes. The Chromalveolata were divided into four major subgroups:

• Cryptophyta
• Haptophyta
• Stramenopiles (or Heterokontophyta)
• Alveolata

Other groups that may be included within, or related to, chromalveolates, are:

• Centrohelida
• Kathablepharidae
• Telonemia

Though several groups, such as the ciliates and the water molds, have lost the ability to photosynthesize, most are autotrophic. All photosynthetic chromalveolates use chlorophylls a and c, and many use accessory pigments. Chromalveolates share similar glyceraldehyde 3-phosphate dehydrogenase proteins.

However, as early as 2005, doubts were being expressed as to whether Chromalveolata was monophyletic, and a review in 2006 noted the lack of evidence for several of the supposed six major eukaryote groups, including the Chromalveolata. In 2012, consensus emerged that the group is not monophyletic. The four original subgroups fall into at least two categories: one comprises the Stramenopiles and the Alveolata, to which the Rhizaria are now usually added to form the SAR group; the other comprises the Cryptophyta and the Haptophyta. A 2010 paper splits the Cryptophyta and Haptophyta; the former are a sister group to the SAR group, the latter cluster with the Archaeplastida (plants in the broad sense). The katablepharids are closely related to the cryptophytes and the telonemids and centrohelids may be related to the haptophytes.

A variety of names have been used for different combinations of the groups formerly thought to make up the Chromalveolata.

• Halvaria: Analyses in 2007 and 2008 agreed that the Stramenopiles and the Alveolata were related, forming a reduced chromalveolate clade, called Halvaria.
• SAR group: The Rhizaria, which were originally not considered to be chromalveolates, belong with the Stramenopiles and Alveolata in many analyses, forming the SAR group, i.e. Halvaria plus Rhizaria.
• Hacrobia The other two groups originally included in Chromalveolata, the Haptophyta and the Cryptophyta, were related in some analyses, forming a clade which has been called Hacrobia. Alternatively, the Hacrobia appeared to be more closely related to the Archaeplastida (plants in the very broad sense), being a sister group in one analysis, and actually nested inside this group in another. (Earlier, Cavalier-Smith had suggested a clade called Corticata for the grouping of all the chromalveolates and the Archaeplastida.) More recently, as noted above, Hacrobia has been split, with the Haptophyta being sister to the SAR group and the Cryptophyta instead related to the Archaeplastida.